Um, well, I have had crushes on people who already had a boy or girlfriend if that suits your question at all. There was a girl I knew who was just perfect to me. She was madly in love with her boyfriend though, so I couldn't do more than dream about being with her. Nothing I offered was better than what he did for her. I had no choice but to forget her and see where else I could belong.
Things happen that are out of our control. We just have to move on and find happiness somewhere else if the future can't make any promises, you know? That is easier said than done, yes... But that's life. You'll never be happy if you don't make changes where they're needed.