if you like her that much you should talk to her, start meeting up with her maybe? Fair enough, you had bad experiences in the past and it changed your academic future but you can't let that ruin your life, you've got to learn from past mistakes and just move on.
Sounds like you have liked this girl for absolutely ages now, surely that must be worth giving a go. Maybe she will even be very supportive when you re-take your A-levels?
I know being cheated on and all the consequences are hard, i've been there myself, but honestly.. you've just got to give other people a chance to prove to you that not everyone is like your ex!