Hi ParanoidAccuser. I am an 'accused person' who was in the same situation as your partner assuming he's never cheated which sounds like the case.
Aside from me, no one told my DH what I'm about to tell you and I think if he heard it from anyone but me, he might not be my ex now. So I would like to tell you how it truly feels to be on the other end of the accusations. This will be a long post because I really want to give you some insight as to what it's like being the accused.
Firstly, I want to start by saying that as a person who would never ever cheat, at first I was a little bit flattered. No one has ever been jealous over me so I found the initial conversations a bit exciting in that someone could be jealous over me. Surely that meant I was truly loved. As a result of my ex's concerns, he tried so much harder to be my everything and I fell madly 'in love' with him after 15 years of just loving him. This is when things were good.
A few months later, things went bad. The constant accusations made it difficult for me to be me. Because mundane things I did on a daily basis were now being questioned, I started changing the way I did things and questioning everything I did.
I could list the types of things I'm talking about but that would be pointless. The point is that when you are in this situation, you can't just do things like you normally do. This is a very painful thing to go through.
After a while, I felt like I was walking on eggshells. Everything I did was misinterpreted and then I started getting comments like "You aren't the person I fell in love with". "It must be because of the affair". "Why are you wearing so much makeup to go the shop?". "Why aren't you making yourself up as much as you normally do?". Everything I did was under scrutiny and I forgot how to be normal. He started saying really disgusting things about me. He didn't say this to other people - he told me exactly what he thought. Sometimes he would sit me down, tell me not to say a word and then he would try to counsel me over my cheating.
I became obsessed with how to fix my DH. I would research for hours because I knew, he would feel really silly when he realised how wrong he was. I would post on Mumsnet. I would be careful not to say the wrong thing around him but it was hard to know what was the right thing. If I said I like a new song, he said it was because my lover must have sent it to me. If I said "Why do guys like ....... (insert any word that guys like)?" it was interpreted as "Why do you and my other man like ......". If he left the house, he would sometimes hide his phone to make audio recordings of what I did when he left the house. The dog snoring = me cheating. His own voice talking to me (muffled on audio but obvious) = me cheating. He put surveillance cameras up. He moved them around. I didn't dare glance around to see where they were as that would be a sign I was cheating. If they caught a shadow late at night (as cameras sometimes do), it wasn't the neighbour's headlights - it was me cheating.
I was in shock. I was depressed. I was hurt. I felt victimised. I felt hopeless. I stopped going out - partially because I decided it was easier and partially because he asked me not to go out without others. He didn't force me to do this but he did make my life difficult if I left for short periods even if just to visit my mum and dad. I changed the way I dressed. I stopped making eye contact because it just hurt too much to see the accusations in his eyes. The more I denied the accusations, the stronger his beliefs became.
We went to a lot of counselling. This helped a little but at the end of the day, no one wanted to be the one to directly tell my DH (who was later diagnosed with Delusional Disorder of the Jealous type) that he was talking crazy. They hinted to him. They talked about domestic violence but because of his mental state, he interpreted it as them telling him he was being abused.
He fixated on a man I had never met in our neighbourhood. He talked to the man's wife and no doubt upset their relationship. The police came to our house and told him he would be charged if he went to their house again as the lady had no doubt her husband wasn't having an affair.
In the end, we were both so miserable. We still loved each other but we also hated each other. He went to a psychiatrist and even then after getting an actual diagnosis, he still believes I am the bad guy in this.
I left 6 months ago. We are still both devastated. I'm devastated because he refuses to talk about it which means it can never be resolved and I will never get his forgiveness even though I didn't do anything wrong. He's devastated because he thinks his partner of 19 years cheated on him and is still to this day, lying to him about it.
There is no easy answer here but if I were you, and you have no concrete proof or evidence of him cheating, I would get help asap. Don't rely on your own ideas about what is cheating and what isn't cheating because sometimes our brains interpret things wrong and we need help to see clearly. Our help came too late and we are both now shells of who we once were. I imagine it will get better for me over time but for him, it may eat at him forever. Probably more so if he ever comes the realisation that he destroyed our marriage (unintentially of course) over something that wasn't real. I put up with it for 2 years and I really wish I hadn't because it upsets me every day. I still wake up crying most mornings. I'm crying while I write this post.
Best of luck to you. I do truly wish someone had told him where his accusations were headed in terms of our relationship.