Heart Muscle Weakness - the Root Problem of My Chronic Heath Condition?

I had a "perfect" day yesterday, did things more than I expected, and did not feel anything wrong until I was ready to sleep - when that familiar heart weakness (actually "malfunction" is a better word) occurred. And this morning I woke up with a "not much better" feeling, realized my only option was to cancel all my Saturday classes. But, the good thing is, with a sudden enlightenment that came from no where, I realized what exactly wrong with my heart - weak heart muscle!

The reason I believe that my heart muscle is the ultimate "culprit" (at least one of major one!) is, for so long time, I've been always struggling  with my limit of energy - in order to feel better, or get my heart stronger, I have to exercise, but if I exercise just a bit over the limit, I got serious trouble. And this limit seemed to be impossible to figure out, if I stopped at the time I really feel tired, it is no doubt too late. So I really should stop before I really feel tired, say 5 or 10 minutes before, but this is a real problem, because I would always feel good before tiredness really comes, how could I tell when is the "5 or 10 minutes before"? Yesterday was an example, I even felt quite good after all day's busy working. This morning, I suddenly thought about how muscle works - it gets sore NOT during exercise, but AFTER exercise. And this resembles exactly how my heart functions. So I think it must be the weak heart muscle who messed up so many things for so long.

Of course, I also have so many other issues than just heart muscle, but I do have my reason to believe this is a major one. For what I know there is no truly a treatment for this problem but exercise. So, as I understood long time ago, if I want live a healthy life, I must walk everyday!