I did something similar, although the situation itself was pretty different. I cheated on my then-boyfriend, EARLY in the 6-year relationship -- mainly because another friend of mine who liked me / I liked him back, started going through a divorce and was really agressive with pursuing me, and my boyfriend had been problematically distant for a few months already, so I thought "why the hell not". Yeah, I'm a prize for doing it, I feel guilty too, still. Especially because I really wasn't INTO the other guy going through the divorce, I just needed the validation that I was attractive. Great, right? I HATE THAT I WAS LIKE THIS. It's ugly, it's embarrassing, it's indicative of a very low self-esteem (which I'd hoped i'd worked through by now), it's shitty.
That divorce-guy ended up doing something equally shitty to me, for some reason, I'm still not sure why he did this -- but he ended up taking me out for drinks one night, and telling me all about the most incredible sex he ever had in his life with some girl he was trying to date (apparently he wasn't trying to date me, just fuck me, which we never actually talked about on any level, because it just was so fast. That's my fault because I could have stopped it, but didn't). He literally then compared this woman's body to mine, assessing that hers was so much better, and then waited for me to, I think, react in some way. But, I was so stunned that anyone would say anything like that to me and be serious, that I was speechless. I literally just uh, finished my drink and left, stunned, and never spoke to the guy again.
I kept this a secret from my boyfriend for the next 4 and a half years. I planned on taking it to the grave and chalking it up as a horrible, horrible mistake on my part, and that I'd never EVER do anything like that to ANYONE ever again -- because I'll tell you this, keeping that secret from my boyfriend took some effort. It took effort because now I had something that I couldn't be open with him about -- and that really impeded on our intimacy in general. And yeah, I know that I created THAT dynamic myself.
BUT. BUT. I'm going to say something here. The reality is, the relationship with my boyfriend was NOT very strong to begin with.
This is very important: if I had felt loved, and fulfilled, and happy, and satisfied, and understood, then I wouldn't have EVER let myself get into that situation with divorce-guy. THAT is the truth.
THIS IS NOT TO SAY that it was my boyfriend's fault that I cheated on him. It was MY responsibility and choice, and I made the wrong choice. I am paying for it, as he's now my ex. HOWEVER.
HOWEVER, the relationship wasn't working. I should have dealt with the reality of that, rather than cheated on him, yes.
My point is that I see you saying this: " I've been drinking heavily, obsessing over the notion that a relationship like this will never happen to me again." (emphasis mine).
Do you mean that stellar, great relationship you had with your girlfriend where you didn't really have feelings for her as strongly as her feelings were for you?
Your relationship wasn't working. Don't forget this. She did all the work in your relationship, you just went along for the ride. It doesn't matter if this was easy for you, and that's why you did it -- it means that the relationship was one-sided, and you're right, it WAS one-sided. It wasn't working. Even though you were kinda floating on by, because she did everything, well that's still not exactly "working".
How do you change your thinking? Put yourself in her shoes. Think about how you treated her. Is this how you want to treat the people you love? Did you actually, really love her? Is it even how you would want to treat people you just like, or people you don't EVEN like? I'm asking you these questions not to be mean, but as real, honest questions that you need to ask yourself, and really be honest with yourself about how you answer them. The way you treat others, the way you treat yourself; the way you THINK about others, the way you think about yourself, and what you're capable of and what you're worth. You have to start thinking about the answers to these questions for yourself, and then you have to start holding yourself to the highest standards that you can. And it takes work.
That's all I really know.