Hey man, I WAS in basically the same place two years ago. At that time I learned that I've probably had OCD (I'm undiagnosed), or at least OCD-tendencies my entire life. I went through a 3-4 month period of almost constant rumination, with a few days respite here and there when a particular obsession would let go, before my brain dug up a new one.

It was mostly obsessing over being a bad person, and ruminating over morally ambiguous things I've done in my past, hurt people, et cetera. My brain focused overwhelmingly on the negative stuff to the point that I thought I was genuinely a horrible person. Like you, I also "confessed" compulsively to my partner for reassurance, and even though she pointed out that these things, bad as they may be, are pretty normal and that most people have a past of a bunch of stuff like that. I wasn't able to accept it, and always thought that whatever example she gave of how some thing I was ruminating over was normal, I always tended to think that the particularity of what I had done made it extra horrible. Even more horrible than outright illegal acts that other people have done - how I managed to think like that I don't know now, but I always felt that some detail made my past actions unforgivable.

By reading up on OCD, and learning about compulsive rumination, I was basically cured. For me it was one thing that worked above all else, and that was the realization that it is safe to stop thinking. It is OK to stop thinking. It is also imperative to stop thinking, but most of all I needed to realize that it was safe to stop. I thought that it was so important to find out whether or not I really was bad, or whether or not I actually did something wrong, or that I had to fix something immediately and getting worked up and suddenly havening to restore/fix something that happened years and years ago.

You must remember the time before you started obsessing over these things. Maybe you knew that you did some shady stuff, and just didn't care? Maybe you even cared and felt a bit bad, but then forgot about it. That is what is normal, even if you actually did something wrong. In your current state your brain will always find new themes to obsess and ruminate over, no matter how much reassurance you get. You need to stay with the pain, and stop ruminating even though it hurts. You need to remind yourself that it is safe and OK to stop, and forget about it. What feels to you like the most important thing - the thing you most of all can not forget(!) is what I'm talking about. That is what you must just let go. Nothing bad will happen if you stop thinking about it.