It sounds silly.. but literally - just do it. You've already escalated to a level where you're thinking you want to ask her out, just do it - the longer you entertain the thought in your head and not taking any action, the more nervous you will be. If she says yes, great - if she says no, it doesn't matter; you're in control, think of it as a loss for the girl and leave it at that. What have you got to lose? If she says no, all she'll think of you as is just another normal guy. After all, you don't need a girlfriend to feel validated or successful. Girls are much more attracted to guys who only answer to themselves in the sense of validation, and it's stuff they will pick up on, in the way you carry yourself and how you project your persona.