Yeah that's very true. I generally don't jump to cheating. I'll typically call out the OP for their fair share of the problem. But in this case, it seems like she's willing to put effort into the relationship and he's being very distant. I'm open to being wrong, but it's my interpretation of the situation.

It's a possibility, but I think it's unlikely since he doesn't seem to be putting up a lot of effort to be considerate of her feelings. Imo, the behavior is more congruent with someone who's begun cheating and so expends their emotional efforts on that person.

If it were something particular that OP were doing, it's my reasoning she would mention that he chastised her about it or that she has some level of insecurity about it and, as such, might be worried it is the cause.

Though one factor she might be overlooking is age. Men and women experience opposite social effects when aging. Women become less desirable, in general, as they become less fertile and looks start to deteriorate. Men, however, become more desirable as they mature since they're seen as more wise and stable as a provider. And that could be a reason he cheats is because he found a younger woman. Happens very often.

What OP doesn't say is as important as what they say. And they didn't say their partner seemed disgusted by them. OP said he gets annoyed by her trying to talk about it.

Classic behavior of someone who's cheating. Be careful.