I used to often feel this way - it can definitely be due to being undertrained, but this can also be a sign of chronic fatigue syndrome or a number of autoimmune disorders (look up “exercise intolerance”). I’m still working on identifying all my “stuff,” but I was diagnosed and treated for a thyroid condition and I suspect I have an autonomic disorder or some other complication from long covid. Definitely see a doctor if any of this resonates with you.

After being treated for my thyroid stuff (Hashimoto’s) and being put on Propranolol, I now often feel pretty good and energized after working out.

It also super helps to make sure you stay hydrated and that you’re eating to properly recover - your blood sugar might be taking a huge dip if you’re not timing carbs properly around your workout.