Sleep maybe. My cognitive function is down for extended periods of time after prolonged exercise. I see no effect on exercising vs my life enjoyability except immediately after exercise I feel happy, because that torture is over and I can resume thinking.
Not sure what types of exercise you are doing or your background (I could be more specific if you shared, feel free to email me) but in general I find most untrained to moderately trained people exercise at too high an intensity most of the time. Feeling like exercise is torture, and having lower cognitive function afterwards are also strong indicators of this.
Most exercise should actually feel quite easy — perhaps an 80/20 split of easy exercise days to hard exercise days is ideal. For example, if you were running you might switch between running and walking to keep your heart rate at a pretty low level (zone 1 or 2) in some sort of structured program that ramps up intensity over time but keeps you in these lower heart rate zones 80% of your exercise time.
For me personally I started enjoying exercise and getting a lot more out of it after making most of it feel "too easy"!
https://github.com/matry
Lik Matry is to Figma as openscad is to traditional CAD (Fusion 360, etc)?
Though that does sound like a huge project to take on!