@yellowgreen,
You have to understand, what you "think" you have is beautiful. To you.
This girl has been with the other guy for (5) years. That's a long time. If she sincerely felt something magical with you, no amount of guilt would make her tell you to go away as that is what she is doing.. Rebounds go like that, ultimately, she is still in love with her boyfriend of 5 years.
Not to be rude but you crossed the boundry big time, by entering her life (it takes two), knowing that she had a boyfriend. You can't win in situations like that. Next time sit back, be a friend, be patient and wait for her to leave.
All you've done is come between two people, that have not had that closure yet, to end their relationship... If he forgives her, all you have done is pushed them back to gether. I suspect, he will never forget and their relationship is doomed in the future...
Take note that you played a part in that loss.
And, please consider, what is not yours is not yours until it becomes yours.....