You are wrong
I have to personally attest that the old adage "once a cheater, always a cheater" is FALSE. It IS possible to go through something like that and remain together, trust me, I know. It takes a big man to forgive a woman for cheating on him because the easy thing to do is to end things and move on. Deciding to stick together is an uphill battle, though. It is not easy.
If you love this girl, you can at least try and work things out. Who knows, you may see that you cannot move past it and work things out...THEN you part ways. At least try.
Cheating is an awful, terrible thing. It is hard on both people. But you two CAN move past it, if it is a mutual desire.
Is she sorry and repentful for what she did? You have to look at how she is acting and reacting.
Honestly, take care.
bunny