The funny thing is, the thing I felt most guilty about for a while was actually something I didn't even do. It's super vague to put it like this, and probably leaves the wrong impression of what it actually was, but I was basically accused of something terrible when I was 16. I still can't bring myself to talk about the details for fear that people won't believe me, even though, funny thing is, I found out years later the accusation wasn't even serious, and was basically a scheme to get money out of people. I knew I'd never do something like that in reality, wouldn't even have the moral capacity for it, but it seriously fucked me up for years. I never tested for it, but I even developed some symptoms of PTSD, which, thankfully are mostly gone since I learned the nature of what was really going on.
The other thing is how I treated a friend of mine a year and a half ago. I was very manipulative and difficult without even realising it. And then she suddenly left for good one day. I keep worrying something like that will happen again, so I have this constant paranoia and guilt about my friends. It's probably why I post about such things so much on this forum, because I tend to try and repress my feelings and avoid saying anything about such things to my friends directly, so I let it out here.