Hey there. I comment sometimes about this sort of thing without details, but I wanted to reach out privately as our situations are kind of close.
My fiance, while he claims it has never been more physical, has had several emotional affairs largely to do with his kink reputation, and has secretly had a girl over to play once. I struggle first of all because part of me is like well, how bad is that in comparison, and also if it's in the past, what's the statute of limitations on cheating? Things have been good since then, so does it invalidate all that? And secondly I struggle to know if that's the whole truth, and always will think he's lying. Always. It's worse because one of those girls lives in our apartment complex.
I've done a lot of reading on /r/survivinginfidelity. The subreddit tends to be a lot of venting. They have a website with good resources.
In order to get past this, I asked my Owner to install an app where I can see his texts. It lets him see mind too, for transperency's sake. I also made us go to counseling. It's helping him understand why he flirts and how it progressed, in our case to do with being afraid he will lose friendships if he doesn't give the girls what they want to some extent. Our relationship depends on those two things. It's also pretty essential in my experience that he cuts off contact with Kate. You, too. He has to show your relationship is more important, and that he wants to regain your trust. She will always be a reminder of this to you.
You have to sit down with him (or email if you're anticonfrontation, which I am) and explain to him he can call it cheating or not, but it is to you regardless and it hurts incredibly that you could entrust him with not only your submission but the rest of your life and he could act like it's meaningless to him. You need to outline what you need him to do, with specific actions, whether they're the above or not. For so long I kept being like "make it better. Please make it better." But he didn't know how. He couldn't. That was on me. And whatever those are, you need to make it clear that it's his choice, but it will definitely show you if he cares about fixing it or not.
It's pretty close to our wedding now, and I have a constant eye out. I am afraid to leave the house if he's here and have considered installing cameras just to be sure everything is better before I marry him. I have forgiven him but I may never trust him again. I think you have to decide if you can live with that in a marriage. As for me, I guess I've decided that despite all this I Love the fuck out of him, and I know patterns repeat themselves and... It may all go tits up. But it still will have been the right choice to marry him right now.
I really wish you luck here. I hope I gave some sort of good advice. Let me know if you need to talk more.