Hmm, you should never be in a relationship just because you don't want to hurt the other person. You have to think about what you want out of life. And being together for 6 years is a really long time when you're so young. You do need to see what's out there... How long were you feeling this way? If a long time, you haven't been being fair to yourself or your girl. You say you haven't done it yet cause it hurts her...well, I think putting her through this can be hurting her more.
You do need to tell her, but do it nicely...a blow like this can really crush a person, especially if you've been feeling this way for a while.
Good luck with it! Keep us posted
Never regret something from your past, everything happens for a reason.