Heart pounding constantly

Hi guys. I've been experiencing heart ponding all day for about a month and a half now. I'm only 19, 170 lbs. I was laying down one night, never have had these symptems before, and it just happened and hasn't stoped for about 2 months now. I have been to the doctors, had an ekg, holter monitor, and an echo and all have come back fine. I have xanax but when I take it, it doesn't seem to take the edge off a whole heck of a lot. It just happens all day every day, my heart rate isn't elevated though. I can feel my heart though my chest, you can see my clothes move up and down with my heart and my stomach and everything. Anyone experience anyting this prolonged and this constant and still had it only have been stress? (which is what all my doctors are telling me). Again, no other symptems. No trouble breathing, chest pain or tightness, etc. (its especially worse when I go from a sitting to a position to a standing one) Thanks guys!