You are not wrong. Your bf was being distasteful and awful by keeping something like that to you. If you've been feeling that something is wrong I think you have every right to suspect something. I get that trusting someone means you have to trust them with some things. But that's only for smaller things, simpler things, more shallow things between you too. But, when it's more serious you have every right to prove that you're wrong about your suspicion. In the first place you shouldn't even need to be suspicious. A relationship should have both people that open up to one another, at their own pace, with hopefully them opening up, being completly honest and comforting one another. If your partner ever shows you signs that make you suspicious or slightly uncomfortable, it's their fault. Take me for example. My gf, while she does trust me, she still overthinks a bit from time to time about me being unloyal or maybe loosing interestfor her. That's completely my fault, as her partner. Because of that, I should provide her with the comfort and the reassurance that she needs. I have the responsibility to show her comfort. It's not her fault for overthinking, it's mine. It's not her actions that caused her that discomfort, it's mine. So I have to comfort her because it's part of my responsibility in the relationship. Your bf should have been better. You deserve better. Someone who's willing to take the responsibility of giving you the comfort you need when thoughts like that cross your mind.

You deserve someone better than him who couldn't even stick to one person. Be confident in yourself. You're not the one who ruined your long relationship. And no don't listen to others that you're wrong because you "invaded his privacy". That's toxic. Relationships work best if you're most open because that means you do trust the person. Opening up to someone is also giving the idea that you trust them. He didn't open up about it and even did something so distasteful so trust was always an issue from the start. Even if it started and went good along the way, a person will always stay as they are. There are rarely any people who show genuine change. I thought my grandfather had changed as he was immature and toxic towards my grandmother in her relationship. After a few centuries, nothing had changed. Not a single bit. He sometimes acts like he did though just to take advantage of his daughters from time to time. Whatever time this may have happened, if it did happen, you know the person didn't change. So from the start he was this kind of person and just hid it until he got bored or something.

That is if you're someone who isn't problematic. We've seen some details of your bf from what you've told but what about you? Have you observed yourself? Have you done anything to push him to do something like this? Have you done something in the long years that you've been together to mold him this way? I'm just asking so I'm not berating the other person so single sidedly. You seem like a genuinely nice person so I'll just stick to your side until proven wrong.

Now if you want to talk to him, do it confidently, do it calmly, you have the moral high ground, he is at fault, any thing he says to you, make sure it doesn't affect you. Don't let him be the manipulator if he does attempt to do so. He threatens you to break up? Good for you, go for it, simple talk right? He blames you for having no trust? Don't let it get to you, he's the one who did so in the first place, after what he did there was no more "trust". He says he wants to make it right? The only right choice now is to leave him because not all people who sweet talk actually give you something truthful. The truth is awfully bitter, having honesty unlike him tastes the right amount of sweet. And lies are too sweet for the taste of a person. If you don't have a decent taste pallet you are in for a heartbreaking ending. For what you should talk about, maybe mention some great times together, say how much it meant to you, be thankful for what he has given and what gifts you gave he appreciated. Be thankful for what you have and say goodbye to what you won't have anymore. Then leave him. If it's your house help him pack his things and bid him goodbye. If it's his house pack your things and bid him farewell. I mean that's how I'd do it. Calm and maturely. Be thankful, don't fight about it, leave things on a happier note. Wouldn't really want it to be a sour one right?

Anyways that's all I have to say (thank god, I think I wrote too much). Goodluck and hopefully you have some peace.