I wouldn't be as concerned about the cheating aspect as I would be about why it took so long for you to find out that she was the instigator of the cheating. Has she deliberately held this information from you, or was it something she thought you had known all along but was a misunderstanding? Also, if she has never in your actual relationship given you any reason to think that she would cheat, and you've talked about it but you still have your misgivings, then it sounds like something YOU need to explore within yourself. Of course no one wants to be with someone who would cheat, but just knowing that she did it to someone else in the past does not mean that she is necessarily more likely to do it to you. So explore what it is about your relationship with her - not her actions BEFORE your relationship - that give you reason to believe that she would possibly cheat (either now or in the future), and see whether that actually leads you somewhere legitimate or if it's an unfounded fear. If it's legitimate, then you should be discussing it with her and hopefully finding a solution together. If it's unfounded, then for the sake of your relationship you gotta work to get past it, because otherwise it's just your mentality that is souring the relationship, not her actions before she was ever with you.
