My ex was like this.
It transpired he was the one who had cheated multiple times.
He was judging me by his own actions and then punishing me for them.
When I discovered what he'd been doing, it was still my fault for making him believe that I had been cheating. He'd apparently done it all as pay back for my perceived cheating.
Apparently, if unhadnt.led him to the constant belief that I had been cheating he'd never have cheated on me. Ergo, I brought it on myself.
He wouldn't believe that he was imagining it, he wouldn't believe he had a mental health issue and wouldn't seek medical intervention. It was all me.
His delusions about my cheating led to me being physically attacked as well as being emotionally and mentally dragged. He even accused me of sleeping with my DMs partner and attacked him too.
I had to end it. But by that point he'd destroyed me and very aspect of my life.
I also discovered I wasn't the only one. He'd done this to partners before me, and again after me. He wasn't satisfied until his paranoia had destroyed the woman he was with then moved onto the next one with a 'woe is me, all women have cheated on me' sob story.