Honestly, if you really do care about her, I would highly advise that you tell her. You know you screwed up, and I'm not trying to sound like an asshole, but if she does decide to leave you upon finding out, she has every right to. Assuming you do feel something for her beyond a superficial relationship, you owe it to her to tell her the truth. If you truly are concerned for her and not wanting to disappoint her/change her, it would be best to just lay the cards out on the table now.
If you (hopefully) knew each other before the relationship, it may be for the best to break it off at least for the time being if that is what she wants. Despite this, she does need to know, and things like this have a way of coming out one way or another. The fact that she has been cheated on in the past shouldn't affect your personal decision, and it definitely should not make you want to "protect" her by not telling her. If you think you're more or less stabbing her in the back now, can you imagine the shitstorm that would happen if she found out a year from now that you cheated on her, after agreeing with her that the other guys are assholes/supporting her with your words about how they didn't deserve her? Even if you have not used those words, or do not intend to, any conversations in which you back her up and tell her that she did not deserve to be cheated on directly or indirectly will only hurt you in the future when she finds out.
My advice, in short, is to just tell her. Yes, it is going to suck, and yes, there is a really good possibility that you might lose her. However, life is not a fairy tale, and there are consequences for your actions. Pray that she stays with you, but if she doesn't do not be surprised, and above all, do not pin this on her. If she does go off on you like a rocket, then back off, then approach her after a while and try to at least build your friendship back up. Even if you do lose her as a partner, the best that you would honestly deserve is having her as a close friend after the wounds have scabbed over. I say this with no malice, but it is your fault in this situation, and you need to grow as a person. You may have already, but be prepared to suffer for actions like this, and your punishment will only get worse over time the more you defend her about her past relationships. Just let it out, and adapt to whatever changes in your life because of it.
Best of luck. We all make mistakes, some a lot bigger than others. You don't deserve her, but you never know. You may get lucky and get to keep her. If nothing else, at least you feel regret over your actions, and for that I hope she doesn't totally abandon you. Let us know what happens, and I'll try to help you in any way I can. Love sucks sometimes, but it is a learning experience. Maybe this is just a road you needed to travel to grow as a person. Feel free to PM/AIM me once shit goes down if you just need someone to vent to, and I hope it all works out in the end.
