<human>: My Fiance and I have been together for 8 years(met in college). The first 5 were great, until I found out that she cheated on me, I was suspicious and checked her computer, found Facebook messages and confronted her about it. She denied cheating until she pretty much realized that I had more than enough evidence and then admitted to what happened(oral one time only). She told me that she was going to tell me eventually but couldn't find the right time since we had been so happy lately. Things were rough for a while, but I eventually forgave her and she promised that it would never happen again and that "she would spend the rest of her life making it up to me"
Almost 3 years later...  
        I recently proposed and about two months after that(now) she wants to check my phone and starts acting strange. I start asking her why and eventually she says that if she told me I would hate her. I keep prodding and eventually she tells me that she slept with a married man (once) about 6 months ago prior to the engagement.  
        I really don't know what to do...I have been with her so long, we don't fight that often and she swears that it will never happen again(even though she did that before...), In the past 6 months, she has stopped hanging out with guys that we aren't both good friends with and started hanging out with girls more often but I don't think I can trust her regardless. She begs and says she will do whatever it takes to have me forgive her.  
        I honestly don't know what to do. We have so much history but shes done this twice. These aren't long episodes of cheating, but she has still cheated twice...  
        I guess i'm just in shock and looking for life advice right now. Has anyone been in this situation and actually had it work out (or not work out)?

Summarize the given text in a few sentences.
<bot>: Fiance has cheated twice , been together 8 years , recently engaged. Keeps apologizing and saying that it will never happen again. Dont know what to do