Down in the commments [1], OP has said they had COVID earlier this year. There's a strong possibility that the symptom OP is experiencing is "brain fog" due to post-viral syndrome.
As a PSA: People are recommending exercise in the comments. If you are experiencing symptoms like OP's, do not start vigorous exercise unless you're sure that post-viral syndrome is not the cause. If you have post-viral syndrome, return to exercise needs to be slow (months) and graded. If you overdo it you are at risk of "post-exertional malaise". I've been dealing with these problems since early last year and it has had a profound impact on my life and work.
If you're experiencing symptoms like OP describes after COVID, talk to a doctor.
I suffer from Post Exertional Malaise but from before Covid. Though Covid has made it worse.
Really a horrible predicament because the more you try, the more you get punished.
Currently thinking about taking a break from running and lifting which is incredibly hard because well, i'm a white collar worker that's already too sedentary.
If you or others have tips to get out of the cycle of crashes please share your tips or sources!
This might not be relevant to you, but a lot of runners/cyclists exercise at 'tempo' - an intensity higher than necessary which mostly generates more fatigue without much benefit.
Thanks for the input, intersting article! This might actually be a worthwhile first step in recovery as i run at quite a fast pace naturally and after covid i often hit a hr of 180bpm as a 30 something year old - maybe i should only walk jog for now.
As far as I am aware, the only way you can exercise with several issues (CFS, POTS) is to exercise at a level below what triggers relapses, and to very gradually increase the amount one does over a period of years.
As a PSA: People are recommending exercise in the comments. If you are experiencing symptoms like OP's, do not start vigorous exercise unless you're sure that post-viral syndrome is not the cause. If you have post-viral syndrome, return to exercise needs to be slow (months) and graded. If you overdo it you are at risk of "post-exertional malaise". I've been dealing with these problems since early last year and it has had a profound impact on my life and work.
If you're experiencing symptoms like OP describes after COVID, talk to a doctor.
[1] https://news.ycombinator.com/item?id=31282305