Experimental Pilot reported fuel starvation inflight resulted in an off airport landing.

Narrative

It was a beautiful summer afternoon and I had planned to fly to a nearby grass airport that I frequently visit for some touch and go's. I fly an Kitfox; which is a short take-off and landing (STOL) aircraft; and like to practice short field ops. I went through my usual pre-flight procedure; checking oil quantity; fuel quantity and quality; and inspecting the aircraft. I estimated I had about 2 hours of fuel on board; which was sufficient for the 1 hour I was planning to fly; plus an hour reserve. On the way to my destination; I decided to do some sightseeing instead. After deviating from my original plan and flying around for a while; my Tocsin CO monitor started to alarm; reading 444ppm for 3 seconds and 0ppm for 10 seconds. I immediately opened both doors to allow airflow through the cockpit while I troubleshot and came up with a plan. The monitor continued to alternate methodically from 444ppm to 0ppm even with the doors open. However; the engine was running smooth and I didn't notice anything out of the ordinary. Regardless; I decided to turn around and head home. On my way back; paranoid that something was wrong with my engine; I decided to land at a nearby county airport. I parked; shutdown; exited; and walked away from the aircraft. The CO monitor continued to alarm with the same readings. I decided to reset the monitor and; upon resetting; it read 0ppm consistently. I inspected the aircraft thoroughly and couldn't find anything wrong. I decided to start the engine again and see if it produced a reading on the monitor. Upon restarting; the engine ran smooth at idle and smooth at high rpm on the ground and the monitor consistently read 0ppm. I then decided it was safe to continue my flight directly home; which was about 15-20 minutes away by air. I took off; climbed to 3500; and headed for my home airport taking note to stay over lots of landing options. After 10 minutes; the engine stumbled... I immediately leveled out and pulled the throttle to idle; at which point the engine smoothed out again. I began looking for a place to land. I noticed a small private airport on the iPad that was within ample gliding distance. However; I couldn't see it from my current position and I know that private airports can be hard to find; especially if surrounded by trees. I decided not to search for the airport and; instead; choose a field that was upwind; uphill; no obstacles; and one that I was 100% confident I could make. I squawked XXXX and [requested priority handling] on XXX.X and gave my location. I then proceeded to lose altitude and setup for an off-airport landing. I made a safe and normal short field landing. No one was hurt and neither aircraft nor field was damaged. After examining the situation further; I realized where I had made a critical mistake. I ran the aircraft out of fuel... but the mistakes came long before the engine quit. Early en-route to my initial planned destination; I decided to deviate from my plan without recalculating how much fuel I would need. Later; after making a precautionary landing and frazzled by the CO monitor situation; I failed to check the fuel before departing. I could have easily re-fueled at the county airport and avoided all of this. I feel like such an idiot because I am usually very diligent; safety minded; and detail oriented when it comes to flying and maintaining my aircraft. I make a point to check and recheck; often finding little things that cause me to ground the aircraft until they are fixed. This time I failed to do so; and put myself in a very dangerous situation. I am very lucky and plan to learn from this mistake. Going further; I plan to do the following; be more diligent about scanning my remaining fuel during flight. Use my smart watch; set an alarm for 45 minutes prior to my estimated total run time based on fuel quantity using the vibrate feature. Practice better decision making inflight by reassessing my entire flight plan; including fuel planning; if I choose to deviate from my original plan. Simply choosing a new destination in my EFB may alert me to problems with my plan such as fuel reserves. Tell my story to everyone that will listen in hopes that others do not make the same mistake I did.