Well, from what you've given us it sounds like confessing isn't going to get her to go out with you. If you're really set on confessing probably the best thing to do would be to say that you've developed feelings and you're not sure if she feels the same, and that you don't want to lose the friendship over it.

Of course, then you have to hang out with her when she tells you about the new guys that she likes, which will make you feel shit (at least, for a while)