Why even cheat on her now? Why not just leave her and be able to do what you want anyway? Then you can do as you please with a free conscience.
Ive been through a VERY similar situation. Best advice, dont look at it as 'lost time'. Look at it as the past. You have had your time together, you had your moments together it was good when it was good but now its time to move on. You hooked up with this girl when you were what 19? Thats a LONG time especially in that period in your life to be with one person. You had your good times, have some self respect and move on.
Revenge will not accomplish anything, and it might get you thrown in jail maybe worse. Use this time/experience as a gauge as what kind of woman she really is. And DONT let her use the excuse that her 'friends made her do it', thats the worst excuse Ive ever heard. I dont wont ask details but it sounds like it was an ongoing thing so she cant exactly use simple excuses like "i was drunk' to explain her actions. There has to be a point in time in which she thought about it, then thought about you and still decided to screw you over. If you ever feel like you want to take her back think about this.