I was once in a situation very similar to that, with a girl that was involoved with one of my close friends (99% of the time getting involved with a friend's ex is a BAD thing). She told me she had feeling for me, and I said that was cool because the feelings were mutual. The next day she told me she changed her mind (still had feeling for my friend, didn't want to 'hurt' me or some tripe like that). Didn't speak to her for several years. I guess I was only upset about it for a month or so afterwards, but damned if I wanted to talk to her. Funny thing is, one day we just started talking again; all was forgiven and we are friends again. The poster above me is correct -- keep being a nice guy as we are a dying breed!
Time and conversation are the keys. Some people need a hell of a lot more time, and nothing will ever be right again since the damage is already done, but you're on the right track. Good luck to you.