How do you relax your heart so it doesn't pound so loud/fast when you are trying to go to sleep?

Meditation: Meditation can be very effective in helping one relax and sleep. Close your eyes and take slow deep breaths, inhaling through your nose and exhaling through your mouth. "hyper-focus" on listening to the sounds of your breaths, visualizing the air molecules entering and leaving your lungs. Practice this daily (or more frequently if necessary) for improving mental clarity and decreasing stress.

Right side: Sleep right side down. Avoid left side down which tends to make it worse. Beyond that, the only other treatment that is likely to be effective is a betablocker - you'll have to see your doctor. Short acting Propranolol in a small dose (e.g. 20 mg) is a good one.