Sophie cheated on my BF.

My head is so messed up with this and I feel so alone :/ I don't even know how to bring this up with him.....

Kaz, I assure you that you are not alone. Your situation is fairly common for people with DID, and in fact for people more generally. Individuals vary a great deal in how jealous they feel, and in what they need and want from a partner. In the end your guy may or may not be okay with this situation. That is for him to determine for himself, in his own way and on his own schedule. You are who you are and the situation is what it is. The best you can do, the best anyone can do, is to be authentic and let our significant others decide for themselves if who we really are is someone they want to be with.

*safe hugs if wanted*
You're definitely not alone, Kaz. Una+ is right. These situations can be common for "normal" people, and are even more common for people with DID. (And personally, it seems like yesterday for me that I was trying to fix things after a situation like this with Kat).

Think of it as diving. It can be scary, you might not want to do it, you might be rethinking your decision to climb up there, you know the water will be cold at first- but you just have to take a deep breath and jump. After the initial plunge, the water won't feel as cold, nothing's scary anymore, and you'll feel accomplished. (I know it's not as easy as diving- unless you have a fear of heights- but sometimes with life you just gotta jump and hope for the best).

When I had to bring up stuff like telling my BF what happened, telling them it might happen again, telling them maybe we should let it happen to a reasonable extent, etc., I'd start with the cliche "I have to talk to you about something" or "We need to talk". Yes, it's cliche, but it would help my BF understand that it's a serious topic and that he should prepare himself for seriousness.

I should probably back up a bit, though. Depending on what communication I had and the situation, there'd be different ways that I'd bring up the topics of stuff like this being a possibility to happen again and possibly coming up with a compromise to let it happen again to a reasonable extent. Sometimes I'd write out what I wanted to say, and then read it to my BF (since I'm not that good talking face-to-face with serious subjects). Other times I'd write out what I wanted to say, then just make mental bullet-points to remember to help me not forget anything. Sometimes Kat would write out her opinions/thoughts on the topic and I'd read it to my BF as an added help to explain Kat's situation and the fact that she has the same rights that I do. Other times I'd simply start babbling like an idiot and it'd take a few tries to get out what I wanted to say. Do things to help prepare yourself to bring this up with your BF. If you need to write out bullet-points or the whole of what you want to say, do it. If you need to draw stuff to help you explain things, do it. If you/Sophie are comfortable with it and if Sophie writes about her thoughts/feelings towards this situation at all, and you need to read your BF some of what Sophie wrote, do it. Do anything to help make this easier on yourself.

After preparing, then I'd say my cliche "We need to talk", and then I'd start by saying stuff like "So, you know how Kat did this...?" or "So, we need to talk about stuff that's been going on" or "I have some ideas about how to handle this whole Kat-thing". Then I'd jump right into it, mainly 'cause I'm a blunt person when it comes to serious topics and I just start laying out everything as it is (not a bad tactic, I might add, but do what works for you). I'll mention that I almost never make eye contact while I talk about this stuff- it's easier for me to focus on something like a table surface or any papers I have or my hoodie sleeve-cuffs or his elbow or something. But I make sure to make eye contact glances, and I'll hold their gaze if they hold mine, so they don't think I'm being shifty-eyed and lying or something. It's just obvious that I'm uncomfortable. (I think that helps, actually. The BF knowing that he's not the only uncomfortable one helps him to relax a bit I think). I say all I need to say, then I look to input from him, kinda like how debates work with one side going then the other. Once he says everything he wants to in reply, that's when we'll start actually discussing things like the idea of letting stuff happen again to a reasonable extent. Sharing ideas, turning down ideas, trying to plan out ideas to see if they'd work, etc.
I know it's hard to believe, but at least for me, it gets easier once you get started.

If you feel comfortable with it, you could even print out parts of the stuff you and others have posted on here (you can print any of my posts, by the way), to help you explain stuff or to help broach the subject without putting the stress on you (like if you gave this to him to read).

Remember you're not alone, Kaz. *more hugs if wanted* You have a safe, supportive place here with people who understand what you're going through. Keep taking deep, calming breaths and just take things one step at a time. Especially with your BF. I hope you find a way of talking to him that works for you and helps make it easier. Feel better soon!