Confused on what to do

i caught my fiancee texting a guy at her work and sending him explicit photos. When i confronted her at first she said she didnt send him a picture and that he sa=tarted talking sexually to her and she just blows it off. I finally got her to tell me the truth after i told her i was leaving. She cried and begged me to stay and said she was stupid and would do anything not to lose me. She also states that she didnt do anything physical but a couple of text messages makes me wonder all in which took place after the sexual talking, in one text the guy says that she still wont talk to him and that she is scared to, she replied back saying that she talked to him everyday and that she stays over at work to avoid drama, his replied back saying "wow that is all you can say" in which she replies back "actions speak louder than words". I feel that maybe it was more than talk or that she had a genuine interest in him. I feel betrayed and lost. I would never do anything like that to her. I know in her past relationships she wasnt always faithful but she was younger and i thought our relationship was good and that she had matured. I cant get her to talk any more about it because she gets mad and says " leave if you are going to leave, i made a mistake and i cant change it". Any advice is appreciated.

Ugh, this is tough. My first question is whether you've seen any other signs that she might be unfaithful to you (or that your relationship is having problems). That would be pretty relevant as to how much you can trust her now. (Also, how trustworthy is she in other ways? If she's never given you a reason to doubt her before now, I'd be more inclined to trust her than if you've had lots of reasons to doubt her.) My other question is, do you know *why* she sent these? That might help to get to the root of it (and help you repair your relationship, assuming you want to do so). If this was a bit of work place flirting that got out of hand (because maybe she doesn't feel sexy enough, or whatever), would that be more forgivable than if she was having a long time affair? It worries me, too, that she's getting mad when you try to talk about it. That doesn't imply trust and it doesn't imply a solid relationship. :(