I Have High Metabolism, Tell Me A Proper Diet.

I was always told my metabolism would slow down, but I am worried its abnormal. I m 22, 6 3 and 170lbs although its all muscle and no fat whatsoever. It doesn t seem to matter how much I eat, i can t gain weight. When I count calories I m generally around 3400/day. I get regular exercise but only a couple hours 2-3times a week and am quite strong/fast though I have asthma . My heart rate is generally aroun 78bpm when I m studying, sitting in bed, sitting in lectures ,etc. Sometimes its 90+ when I m doing nothing. Standing up makes it go way up, so @17y.o. the doctor told me to drink more juice to keep my b.p. up. Also, a cup of coffee often gives me heart palpatations, sometimes for over a couple of hours which I find disconcerting so I only drink a cup of tea in the mornings. Is this still normal? I feel like my heart is going to wear itself out before its time. My 20y.o. girlfriends heart only beats about 50-60 times per minute so its quite a contrast.