Have NC'd as my usual moniker is very telling.
Backstory: I moved in with MIL and FIL because of DP's work. We moved when DS was 2 months old. My family are nowhere near, not many friends even after 18 months of being here. Also a language barrier as most people here speak [as a primary] a language I don't know, but am learning. They're happy with English, but it's still awkward. I had PND badly, really suffered with it but am out of the woods now.
In the past I have found sexually explicit texts/e-mails/Facebook messages from DP to a man, who is a friend from school days. I was understandably distraught, but was assured it wouldn't happen again and that he was just trying to deal with the strain of being a new parent and being in a new job and having to deal with my PND. He also assured me that he was not gay, and that it was "all talk and no action" if you get my drift? He cut contact with this person voluntarily.
About 5 months after this happened, I accidentally saw a Facebook conversation with a girl, as our Google Chrome opens the last viewed page. In it, there were some fairly sexually explicit messages, mostly from her but some from him.
She also was incredibly cruel about me, claiming that I was using my PND as an excuse to laze about and that a year after the birth of my child is too long to be clinging to my excess weight. Although DP stood up for me in that instance, I couldn't ignore the messages. We began a trial separation and I eventually accepted him back after a fortnight of him trying his utmost to make things right between us.
Since then, we've been closer than ever, our DS has grown and I've never felt more happy. Our sexlife was back on track and it felt like a whole new relationship.
Fast forward another 7 months to today. He's been texting his very close friend [S] a lot, because she lives far away and so we only see her sporadically. They are very close, have been friends since they were little, and is considered an integral part of our friendship circle. They had a brief fling about 9 years ago, when they were in their teens, but nothing ever came of it and DP was upfront and honest about there being no feelings involved, and that it was just sex and "not very good sex at that" [his words].
His phone vibrates and because I'm closest, I check. It's from S and it is filthy. I go and check his previous contact with S only to find around a days worth of messages, but all the messages have turned deeply sexual.
DP - "You can tie me to your bed and do anything you like xxx"
DP - "TwoSugars is having a good week - we've had sex three times but I still want more can you come and finish me off"
DP - "Sat here wanking to porn while TwoSugars is asleep, wish you were here"
I've confronted him, and he has pulled his usual sadface and is there looking miserable. I have tried to keep my cool and have told him exactly how disappointed I am, that I thought we were OK and that I've never been so disgusted. Because she's such a close friend I now have to wonder what has been going on, whether this is new or whether it's been going on longer because he's known to delete messages from his phone as he likes "a tidy inbox".
For various reasons regarding money, my family, his family and commitments, I cannot just up and leave, either with DS or without him. I have no idea what I am meant to do about this. I'm so ashamed of myself. I thought we were OK.
I'm so sorry it's long, but I just didn't want to dripfeed. I mean it when I say that I cannot leave, even as a break. What do I do?