You do not wait for a woman who has a boyfriend, it does not compute.
I think I agree, you are her backup...this is not good for you in any way. Also, cheating carries extra excitement, if someone isn't bothered by the guilt, they get that extra kick of excitement when they cheat...again, not a healthy thing to try.
In fact, if your goal is to be with her, getting on with your life, even getting a girlfriend, is still the better play for you.
Or, just tell her you want to date her if you wants to break up. Be blunt, and nice, no pleading
In the case that she's just scared of leaving what she has, she needs a kick to get her off the fence.
First loves thought..always hard. But keep in mind that's because its a first, not because its more "real or better" in the realistic sense. Emotionally though, that first love always gets special treatment...just how our brains work. You could always find a way to fight with her and hate her, it may clear out those feelings
(kidding, that's not my advice...at least I don't think it is!)