Wocket,
What you're dealing with here is complex and you are doing the correct thing by sharing your frustrations in order to work through them.
Even when you recognize that a thought or concept may be both causing you pain and being logically flawed it can be hard to let go of. Why? b/c emotions are stubborn motherfuckers and when they're all fucked sideways and shook the hell up like they are whenever you've been traumatized they bump into each other and push and pull each other and generally make it a confusing headspace to be. That's how it's always been for me at least.
On the guilt that shouldn't be your's to own thing, you were getting at in the below quote. I know you know and see it that it isn't your fault but again emotions dig in hard and don't want to let go of things (I'll get to where I think they may become from after).
A big regret I have is not objecting to her leaving the bar right as she left. I don’t think I would have allowed it to happen were I not as drunk as I was. I am very angry with myself for not doing this. Really angry. I know what happened afterwards is not my fault, but the fact there was something under my power and control that I could have done to stop it really, really bothers me right now.
While it doesn't make "sense" that you would feel guilty it does make sense from an emotional standpoint as to why you would want to feel guilty.
I know, I know "Why in the fuck would anyone want to be guilty, asshole?"
...would have once been my response to that. I know b/c it happened but I was wrong. Maybe this next part will apply directly to you.
The desire for me to feel guilt was actually driven by my fear and desire to feel safe and trusting again (I too was always a pretty trusting person and felt I had lost that). So that fear and that desire wanted for me to take responsibility for what happened. It made me want to change that last sentence fragment to add in "let" so that I would be able to say...
"Well DAAAMMMNN no wonder this happened to me I see my mistake now and how I can be SAFE again in the future."
Then there was the follow-up response to that
which was:
"I am going to need to beat myself to shit with this terrible thing that I've done and/or allowed to happen so that I never allow it to happen again."
So that "non-thought" (but really just primal and partially or entirely subconscious thought) process driven by emotions was getting in the way of allowing me to let go of the guilt. Once I kind of got myself to this spot I was able to start pulling it apart and then slowly started to let go of some of that anger. There was and is still plenty of that coming from other aspects of this shitstorm but that desire to exert control over the past and regain safety in that way have both gone by the wayside.
I still have thoughts that I might be guilty or somehow I deserved what happend and they will pop up every now and again (and someone call me if you can figure out how to make THAT shit stop) but I recognize that it's just a thought and it will be on it's way shortly and it doesn't affect me as much since the emotional weight such thoughts had previously carried has been lifted. So slowly those thoughts have come around less and less and I know eventually they'll be gone completely.
.....and just for a moment let's get a time machine one-use-only and go back to that night and you to prevent her from leaving the bar.
What happens?
Sure, maybe you are happy in the NOW of this new timeline we've created but the broken part of her that allowed her to do what she did in that now defunct timeline isn't fixed. It's still in the NOW of the new timeline.
Now from there can you see another scenario developing that will see have that broken part reveal itself and then you wind up being betrayed again?
What if she goes out for a drink after work?
Or when you leave for a work trip?
At a bachelorette party?
Can you see a similar situation to the old timeline occurring again?
I can b/c that part of her is still broken.
Since you said you were still thinking of R and if you decide that she has done enough to allow for you to feel safe with her as a partner and done enough to for you to feel that she will continue to work to fix that broken part forever that's a bit different.
So I'm saying even though you had no hope of preventing this because it was something inside of her that caused it, that doesn't mean there isn't a chance that you won't be able to trust that she will be able to prevent it herself in the future and that she will choose to do so.
Stay Strong,