This can be accomplished.
You just need to deny it. Do you wanna screw her or just get off? I suggest just getting a BJ, and maybe blow all over her face then tell her to beat it. Maybe have her blow you in the car then push her out at a Mcdonalds or something. No way she will tell your GF that and you can get back at her for being a bitch all those years.
Whatever you do make damn sure there are no phone pics, text messages, etc. Then just deny everything. It's your word against hers.