Never married, but I'm not some naive kid either. Love it when people act like that's some sort of insult. People often get married in a rush only to get divorced 2 years later. Marriage is not the end goal in my relationships, and I'm currently in a 9 year one. I've been in a few where I've been suspicious but you know what I did? Had a conversation about my concerns and why I felt that way. If I was looking to end a marriage, I would seek my lawyers advice and probably see a therapist. Divorce can be a traumatic event and anyone can benefit from help processing it.

When the division of assets is at stake, I can agree that the rules change a little. I've witnessed first hand how the obsession of finding proof has ruined a friend with proving his wife was cheating. He got it, and he tells just about everyone he meets. Our friends don't really like being around him because he feels like he needs to tell people about how awful his ex-wife was. This was 12 years ago. The way he talks about it, you would swear the wounds are fresh. He really needs to see a therapist, but refuses. I've got a few more similar stories which help shape my view. Those who have come out of it without visible emotional damage are those who have sought help.

You do whatever you have to do, but my experience is that the obsession is very unflattering and depending on how it manifests may affect existing and future relationships.

Seems like a lot of unnecessary stress to put yourself through, in my opinion. I say just move on. You don't trust the person so why try to remain friends? Seems like you're just providing them more opportunities to manipulate you. You don't need to warn the world of their malicious behavior. It's not really a good look to carry around that baggage, whether or not it is justified. A well adjusted adult will move on.

You don't need evidence to break up. Sometimes it's better to walk away. Cheaters are going to continue to cheat, and confronting them with evidence isn't going to change anything. If you still feel like you can't trust them after talking about it, maybe it's time to break up. Healthy relationships require trust.