Does exercising too much sometimes lead to "crashes" that last for days such that it is impossible or very hard to even predict on what day the crash will end -- crashes severe enough that you cannot meet your work, social or family obligations sometimes? If so, you have a mild case of chronic-fatigue syndrome, and I can point you in the right direction.
Not my case, I'm simply lacking the stamina to exercise but push myself through cardio nonetheless. Maybe I need to do a blood test or something like that. I was hoping for some leads on what could cause this.