Should I tell her how I feel or no?

i think she knows, sorry. you asking her to prom was a clue but girls generally feel this and create distance as a result. she has a long-term boyfriend; it's just not your time and place. there are other girls out there and even a bunch that will not only say "don't be sad. if you need anything, I'll be here to talk to you" but mean it. now go find 'em.