sorry you have to go through this am sure it broke your heart. cheating is the worst thing that could happen in a relation. She seems like a good lady but what she did has no excuse and she knew it was wrong and she continued doing it (am sorry to say that but she didn't cheat once it wasn't a one night stand) what if you haven't found out? how long will she continue to see him?
for me cheating is unforgivable at least on the short term and continuing with your current relation will only deepen your scar, have you considered taking some time apart from each other? think things through and give yourself time to heal?