@Ciscostudent,
Quote: I can't just tell her because I'm afraid of getting rejected.
If you never approach anyone due to a fear of rejection, then you will indeed live a long, lonely life. You need to get past this fear of rejection because life is filled with rejections. Basically, if you never put yourself out there and ask, you will never know. And rejection hurts for a lot less time than regret.
As for asking this specific girl out, if you know she has a boyfriend, then you probably should not ask nor should you continue to flirt. But, having a boyfriend and having a fiancee/husband are two different things. So I see no reason that you cannot drop a hint that you would sure like to take her out "if not for the fact that you have a boyfriend." Get the idea? But don't wait around for her.