Ugh. This is a hard one for me. I've had two major relationships after graduate school. One, a 17-year marriage. The other, a 10-year partnership. Children in both. What I, myself, learned from those nearly thirty years is that people sometimes get a little bored or dissatisfied in these very long relationships. It is quite easy to take for granted someone who sort of seems to have been there next to you in the bed almost forever. And when, either by chance or design, another person shows the slightly bored or maybe dissatisfied party some newfound attention, well, that can be pretty darn intoxicating. I mean, literally, like a drug.

So, my concern for you, as you mention that things are not in a good place with him right now, is that she may begin to shower him with some attention that appeals to his sense of whatever he has decided is not exactly workin' with you at the moment, and that he concludes he really, really likes the way that feels. If and when that should happen, he could, based on my experiences, be off to the races.

So, my counsel to you is to nip this in the bud now. Address whatever your joint issues are head-on, including going for couples' therapy, if needed. Be wary of secret texting and messaging, and other online activity. I am not trying to catastrophize here. I saw all these things when I was going through this (both women I referred to cheated on me), but I never spoke up until it was so in my face that a 2 year-old could have seen it. Don't do that. Lay down the law now. What will you do should he ever cheat? Will you leave? If yes, tell him that now. If no, well, then, your day just got a lot more complicated.

Knowing what I know now, I would not forbid him, were I in your shoes, from interacting with her at this point. But I would closely monitor his behavior. Contrary to popular notion, people can def cheat without having sex. The hallmark of cheating is not sex, it's deception. So, if you start catching him in stupid lies and he's still seeing her, you are being cheated upon.

Sending you positive vibes and best wishes. Take care of yourself