Air taxi pilot reported during arrival in weather experiencing windshear resulting in loss of control of the aircraft. The pilot was able to regain control of the aircraft and landed safely.

Narrative

I was flying the RNAV (GPS) RWY-XX approach to ZZZZ and experienced a windshear or heavy rainfall/downdraft event during final approach that caused a slow-speed situation. The recovery of this condition resulted in unplanned altitude loss and loss of situational awareness from the Final Approach Fix (FAF) to the Minimum Descent Altitude (MDA). A regional weather system had entered the area and brought with it widespread significant weather. This included moderate to severe rain cells and surface winds in ZZZZ gusting to 35 to 37 knots. I departed ZZZZ1 IFR on RWY XX and on the Gateway Departure. (I was in almost persistent IMC from about 1;800 feet and until I was around the MDA in ZZZZ.) I checked in with Approach and they immediately gave me vectors to avoid weather cells nearby. They also maneuvered me around the most significant weather of the region that was stationary and located to the northeast of ZZZZ. The controllers did an excellent job. About 5 hours earlier; and on my previous trip to ZZZZ; I had been given 'all deviations for weather approved; just let me know when you are direct ZZZZZ' from Approach due to the same weather.For this afternoon trip; the airport had changed runway directions due to surface winds and this meant that the approach corridor was now in the most intense weather of the region. It was also now in the choke point for inbound traffic. I was given delay vectors in order to flow faster moving jet traffic in front of me on the RNAV-XX. My aircraft was now in moderate turbulence and rainfall at this point in time.When it was my turn to start final approach; I was cleared by ZZZZ to 'cross ZZZZZ1 (FAF) at or above 1;600 feet; you're number 2 following a B737; caution wake turbulence; cleared for the RNAV-XX approach.' I was in solid IMC and in moderate precipitation and continuous clouds. I began my descent from 3000 feet and remained on Autopilot. (I had engaged AP (Autopilot) at 1;000 feet out of ZZZZ1; and it had stayed on until I initiated my event recovery.) I also noted that I was now flying into yellow; dark yellow and light orange NEXRAD (Next Generation Weather Radar) returns. My weather radar was on as I had been switching between the two to validate NEXRAD.Prior to arriving at the FAF at ZZZZZ1; the aircraft leveled off and I brought power up to 60% to maintain approach airspeed of 120 kts and altitude. At this time the rainfall became severe and the passengers sitting immediately behind me became verbal and started comforting each other. I then noticed that the AP was pitching up to 10 degrees and that airspeed was decaying. I applied 90% power and then 100% power within about 5 to 10 seconds of each other. As the airspeed decayed below 100 kts; I disengaged the AP and began my recovery. I brought the pitch angle below the horizon but I was still losing altitude and airspeed wasn't increasing. The lowest airspeed I remember seeing was 87 kts. (Vref for this flight was 85 kts; with 8 passengers and over 300 pounds of bags.) I brought the nose of the aircraft down further to gain more airspeed which finally began to increase closer to the normal approach speed of 120 kts.During this time; I was focused on airspeed recovery and my situational awareness was low. I then noticed white capped waves out my window and this triggered me to look at my altimeter. (The majority of ZZZZ approaches are over the city while landing to the east on RWY XY or XZ. I've been in the area for 2.5 months and this was my first landing to the west and seeing any ocean waves at all; and from this height; was new to me.) My altimeter indicated that I was around 800 feet and my GPS indicated that I was 2.2 miles from RWY XX. I did not have the airport or airport landing environment in sight and was in and out of a scattered layer. I also did not have an advisory glide path on the G1000 PFD (Primary Flight Display); despite having VTF (Vectors to Final) activated and APR armed and displayed on both the MCP (Mode Control Panel) and in the PFD status board. I did; however; have a yellow GP (Guidance Panel) box on the PFD but it did not have any additional information or advisory GP.With my airspeed recovered; I corrected my course for the 35 kt crosswind from the right. The G1000 was indicating around a single dot deflection left of course. I continued my descent down to the MDA of 390 feet. (The published is 384' and set as 390' in the Garmin.) I went through a small cloud layer and saw the entire airport environment. Visibility was good at this point and I could see about 5 miles. Also; during this landing approach; the Tower Controller announced twice that 'winds gusting 35 kts.'Once I had the runway environment in sight; I had to initiate a crab angle of about 45 degrees to get back on course and then again to maintain runway centerline. The crosswind landing component was calculated at 22.5 kts (Wind 300 degrees; gusts 35 kts; RWY XX) but the wind was also variable by 30 degrees and thus could have been higher. I lined up on the runway and kept about 70% power in to maintain centerline. I attempted to trim the airplane for the crab but instead transitioned to a forward slip and tried to give myself a little help with aileron trim. There didn't appear to be enough trim so I kept the power in; set flaps to Take Off; and used both of my hands to keep my right wing into the wind. I landed at around 100 kts and on one wheel. The plane quickly settled in on all three gear legs and rollout was uneventful. (In ZZZZ; there is a row of 100 foot trees that line almost the entire right side of RWY XX and provide some wind protection at the surface level.)ZZZZ Tower asked me for a report on flight conditions and in the immediate moment; the only explanation I had for my airspeed loss was windshear. I reported 'windshear on final' and they seemed to appreciate the information. I don't think there had been any approach PIREPS for the changing weather. In the following hour; multiple airliners would end up going around and I watched an A320 bounce a landing (or possibly get picked up by the wind) then attempt to recover the landing and then ultimately go-around. My loss of situational awareness was caused by the slow-speed event and weather near the FAF. I focused on airspeed recovery by applying power and decreasing pitch angle. I should have followed the recovery with a 'missed approach' callout to myself and completed a complete 'go around.' Reflecting on the event; I know that I was pretty rattled by the airspeed event; unplanned altitude loss; turbulence and surface wind challenge. I was emotionally charged to just get the airplane safely on the ground and to not have to go missed and fly back through the significant weather (by myself) that I had just flown through.During single-pilot operations; brief each approach with a special emphasis on the missed approach and on the conditions that could trigger the missed. This should be tailored to the specific challenges or threats that are present on each flight. This will pre-load the mindset in a similar way to an aborted take-off. For example; 'If we lose an engine or have any abnormality prior to rotation; power to idle; apply brakes; taxi off the runway and debrief.' During the approach to landing; 'If we fly through the yellow and light orange weather returns; and this creates an unstable approach; either by airspeed or altitude loss; then we will go missed; TOGA; full-power; pitch to the command bars; clean up; fly the missed and either try this approach again; hold for the weather to improve; or go to an alternate.'- A second pilot/First Officer is always valuable and should be on every flight. From my perspective; we are only one of a few operators on the ZZZZ routes flying single-pilot passenger service. (Another company operates with First Officers.) I know that highly experienced Company Captain's operate single pilot each day and with a high margin of safety. However; for a relatively inexperienced Captain like myself; even an entry level First Officer would greatly increase the safety margins by actively monitoring aircraft status and catching things that an often tired and overwhelmed junior Captain is missing.- I strongly believe in a seasoning process for newly certificated; direct-entry Captain's with no previous operational line experience. In contrast; the Captains who have upgraded from the First Officer ranks at the company often have over 1;100 hours of operational experience; including all the weather experiences that go with it. This is a very valuable process and results in a very safe; competent and confident Captain. I've seen it myself and have been truly impressed with the results. I can name the faces of the FO's that 'trained me' and down here in this region. The current requirement is for a First Officer to be present for the first 100 hours in make and model; but only if the flight is being operated under IFR. I think that for most people like myself; that number should be 250 to 300 hours; and the FO should be mandatory. There should also be a regionally focused seasoning of some sorts. Similar to my experiences here and in the US; we should pair new Captain's in a region with First Officers that has been 'in theater' for a few months; to expose the Captain to the weather and operational aspects of each region or the areas they will fly their assigned routes. As of right now; this regional adjustment period is managed by Operations and by their own motivation and efforts to do so. They really helped me out when I was first in the area and while TDY (Temporary Duty) in the US; but it was always at the discretion and action of a specific individual to get me more experience. I think it should be a program or set of rules; and not by individual effort or variation.