I'll do my best to be concise. My wife known about her BPD since she was around 16 or 17. We've been together for 4 years and got married in June of 2017. 

One of her co-workers also has some similar mental health issues and they bonded over it and began talking to each other in a way that I would describe as very sentimental. I spoke to her about my unease with the nature of their texts and she said it was just a way to pump her self esteem up, though I was still not totally comfortable I know she has issues with self worth and I don't want her to feel worthless so I shut up about it because truth be told I was happy she had a friend to confide in even though I secretly wished she would come to me for that type of support (not necessarily instead of him, but as well as him).

Fast forward to 2 weeks ago, I'm using her phone to play music on spotify and he sends her a text that says "I can still smell you in my sheets". I'm very alarmed and ask her about it, she explains his partner just left town and he probably sent it to my wife by mistake. I believe her although I'm distraught over it the rest of the day. 

That night I get insecure and look through their message history (not my proudest moment) and there are a lot of really mushy texts but nothing to be worried about really. Then I remember she uses Instagram sometimes to message people, and it's there that I come across very sexually explicit texts and one saying that meeting him was the best thing to happen to her in 2017. I'm crushed.

I confronted her about it all and she told me that she was sorry but nothing happened between them, it was just a crush and nothing to worry about. I spent a night at my brothers and came back ready to work it out. That's when I saw they were still messaging. I told her it needed to stop, she agreed. Things were okay for a few days, and then there were more sexually overt messages. 

Finally I'm fed up, I go to her work to confront the guy who won't respect the fact that I'm married to the woman I love and upon confronting him he is shocked to discover that our marriage is not open. She had led him to believe we were in an open relationship and that I was okay with everything. He said anything I needed to know he would answer honestly.

Turns out they had hooked up a few times, the message about him smelling her on his sheets was indeed intended for her, and up to this point he was under the impression we had a healthy open marriage.

I spoke with her after all was said and done and asked why. Reasons she gave me were that I didn't do enough housework and she felt underappreciated for the work she did, that my libido is considerably higher that hers and it made her feel inadequate so she went outside the marriage to feel more like she could satisfy a person's sex drive, and that I'm boring and the newness of everything was gone.

Now I'm here picking up the pieces and trying to decide if I can learn to trust her again, if I'm willing to put in the work, toand what I could have done to be a better husband.