As one bisexual to another... welcome.
I think you have far less to risk if rejected by the girl than you do by the boy. I remember being foolish enough to pursue crushes on girls when I was in school and needless to say, the climate towards me became rather hostile. Might be best to take no more interest in the guy.
If she didn't have feelings for you after all that time, and even ignored you for a whole year on the suspicion that you liked her, then it would seem that confessing now would yield no different answer. Take a chance if you'd like, but I seriously doubt that she will have similar feelings for you.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen