Here's a better idea. IF you want any chance of getting back together with this girl, you can only have sex with women she has ZERO chance of finding out about for the next three months (I say three months because after that, fuck her...that's too long a break to expect an abstinence). If you do get back with her before 3 months are up, and she looks you in the eye and asks if you've been with anyone in the interim...you tell her that crap you said earlier about how you couldn't possibly because your heart was still with her. That's right: you will need to lie, and very convincingly. Trust me on this...she will NOT be "understanding and mature" about it if you bang another girl in the near future.
(For those who will give me crap about this, remember the Ross/Rachael drama on Friends when he did a girl when they were 'on a break'.)