Hugs Ellie-oop
That sounds truly awful and I'm not surprised that you are still haunted by it.
I've had much less traumatic experiences that have haunted me for years afterwards.

It sounds like you've already gotten yourself through several stages of recovering from it. Well done!
I hope talking about it here helps you to recover even more.
You might want to give talking to someone irl another go at some point as well. I'd imagine that things have really changed since it first happened both the state you're in and what might help you.
Depending on where you are I might be able to recommend some people, if you're interested pm me.
Maybe you could talk to your dp about it at some point, or about some parts of it (if that's possible). It talk me a while to talk to my dp about when I was assaulted (and he was even at the party with me when it happened, just not close by). It then took him a while to calm down and be ready to talk to me about it but after that it was really helpful, also just helped that I didn't feel like I was hiding it from him anymore. Of course that's just my experience which is totally different from yours.

Hugs again. You really have nothing to blame yourself for.
It doesn't seem like it should be but I know that it's actually really hard to leave abusive situations.

Good luck recovering and coming to terms with this more, it's a long process and it seems like you're doing really damn well so far.