Guy trouble question?

Last year, there was a guy who liked me, but I didn't like him. Toward the end of the year, I started liking him but didn't do anything because I was scared. Now he has a girlfriend and I don't know what to do. I like him so much and I feel so stupid for missing my chance. It hurts to see them together, but I can't tell him how I feel now since he's dating someone else. A huge part of me wants them to break up so I can be with him, but I really like the girl he's dating and don't want her to get hurt. How do I deal with this? I feel so powerless, hoping something might happen between us, but knowing that he's with her so it can't. It really hurts me and I need some advice. Please help?