H4run
Deity
You should also consider not to fudge her up as well. If you can snap her through manipulation tactics and make her do things that she actually abhors like following her whims during her weakest moment and make her do things that she is against morally like betraying her bf, this would damage her psychologically and change her for good, is your accomplishment worth it if that is the price? and will you ever trust her after her betrayal? will she ever trust and love herself after what she did?Well, I want to know how to become her boyfriend, but I don't want to fudge it up.
I got a scenario that runs exactly like that, and it's gone dirty to all parties, basically, my nephew screwing up with a nice girl that had a bf, my nephew is really experienced and slick, I warned both of them, but he's just don't care about others and doing his craft really well, but that messed her up really badly, and you don't want that to happened if you love her, or if there's any emphaty left within you as a human being right? because my nephew grow up in an abusive house that made him manipulative and becomes who he is.
Just taking it easy. control yourself, and take it slowly, no rush, you don't want to be too eager and like Narz said you don't want to be taken for granted which made winning you mean nothing. If you are serious with her, you should consider also how the process will not harm her well-being and psychology, make and realize that this is not all about you, if you can't consider other actors at least consider her and her bf as well, you might not like him now but he can be you one day and there might be another man want to screw her up in front of you, and again you don't want that, don't you?.
In the meantime while she's in a relationship, as you are still young as well, open yourself for other opportunities as well, she's not going anywhere she got her bf and you can have one also, and you can maintain your relationship with her and play a long run game and wait and see in the future, perhaps you'll change your mind, perhaps you will be glad the relationship never happened, or perhaps it will happen but at the right sweetest moment. However you don't want to sabotage her relationship from within, that's really low.