A C182 pilot flying through rain showers reported temporary loss of control due to downdrafts.

Narrative

Root cause was inexperience in conditions of flight. Vectoring around rain showers and virga downdrafts; first time flying into ZZZ airspace; not able to locate the airport visually; experiencing an uncommanded descent.I was arriving ZZZ1 airport for a course. ATC was helping me divert around rain showers as I entered ZZZ Bravo airspace. As I was approaching ZZZ1 with the intention of landing I was unable to visually locate the airport even though ATC was directing my eyes and I could see my relative position to it on the GPS. I began to experience a loss of altitude at or about a 300 to 400 FPM descent which I initially tried to raise the nose to climb. Manifold pressure was on full. As I worked the problem ATC then turned me south to keep me away from an obstacle which I complied with still trying to understand why I could not climb. I tried to lean the mixture for more power as I thought density altitude might be the problem and became concerned about airspeed as I kept trying to raise the nose. Eventually I pushed the propeller full forward which quickly restored a climb capability of approximately 500 feet per minute and climbed from about 7500 feet to 8000 as assigned. I asked for vectors to RNAV XXR approach to help get my eyes on the airport which was successful and then asked for the Visual Approach.My stress level was moderate coming in to the Bravo airspace as I did not have much experience being vectored around active rain showers and was concerned for turbulence based on PIREPS I heard on the radio. Stress was added since I could not visually see an airport I knew to be rather large and felt I should have seen it already; and additional stress when the downdraft and loss of altitude began. I believe the correction to the problem is to fly with an instructor on a number of occasions in marginal or actual IMC conditions to gain more real world weather related experience. I also gained knowledge on how to deal with downdrafts and high altitude flying having attended the course this weekend. Additionally configuring the airplane for max performance (full RPM; appropriate mixture etc) early before entering an area of mixed or unpredictable weather early to have the airplane ready for max performance should the circumstances demand it.