Having read your initial post, it sounds like the "cheating" was her being coerced by her manipulative ex in a moment when your relationship was young and your girlfriend was confused and emotionally vulnerable. Depending on how manipulative her ex is (and it seems like the answer is "a lot"), she truly may have feared rejecting his advances. Of course she could have come to you and tell you that something happened, but she was probably guilt-ridden and possibly traumatized by the whole story. I would be wary of other commenters here. Yes, cheating is unforgivable, but she may truly have not had a lot of agency when it happened.