Remember, be sincere, that's very important.
If she says no, I know it will feel like the end of the world, but really, you'll live. We've all been through the pain of heart break, some of us more than once. ;)
Just talk to her, really, don't be scared, just show her that you mean what you are saying, and tell her everything you've said here.
Yes, she could turn you down, but she could also take you back. You won't know until you ask. :)