I’ve yet to read a satisfactory answer to this question from anybody. The reality is, this really effing sucks. You can’t get the girl to like you and you can’t stop liking her. Most people are going to tell you to find another girl, but that’s only marginally easier than the other two courses of action.
You could “confess” your feelings to her, that will get everything out in the open, but nine times out of ten it won’t make her dump her boyfriend for you.
This is a predicament, a ridiculously common one. Predicaments are problems that don’t have solutions. You have to manage the situation because you cannot solve it. Read a lot of books, watch a lot of TV. Watching other characters in the same situation as you will help you understand and work through the issues.
The first girl I fell in love with does not love me back. At least, not the way I want her to love me. The more I think and analyze it, the more I realize that she just doesn’t think about love the same way I do. It’s the hardest thing I’ve ever had to deal with in my whole life, and I’ve had to deal with a lot of hard things.
Sorry you have to go through this my friend. You will come out of this a wiser person, I guarantee.