When I fail to sleep the normal time I can't fall asleep for the rest of the night, I start feeling my heartbeats pounding when laying down, why?

Cardiac awareness. The name of you symptom is "cardiac awareness." If tends to be more prominent if you lie with your left side down. Shifting your position usually helps. Try reading in bed before you turn out the lights and you may find that it both relaxes you and prepares your mind for sleep. (and there are no unpleasant side-effects!). (unless you read about vampires).