Stuck in my head. **Trigger warning- mentions of violence**

Hi all,
I have slowly realised I have an issue with ruminating over things I have or haven't done. In this case my ruminations are centred over something that I did at some point in my mid-teens. I am in my early 20s presently. I do not have an official diagnosis for OCD, however, I plan on seeking out a psychiatrist as I believe I am possibly experiencing Real Event OCD.
I will get these thoughts that just randomly pop up when I do my day to day business. My mind automatically goes to that past event.

Today, I was ruminating about at what point should someone be written off for life based off of something they regret because they have harmed someone. I entertained this thinking for about an hour where my mind was echoing extreme views about who deserved to be exec*ted or brutally t*rtured and it left me with a fearful and upsetting sensation in my head and stomach. I have repeatedly had such kinds of thoughts. I imagine that yes, people make mistakes, but I get this really uncomfortable and sickening feeling when I have to analyse at where the line is that a mistake becomes someonething more than that. I have to analyse this thinking otherwise it will drive me insane. It is like scratching an itch.

I am confused about some of the things my stupid mind is doing. I have a habit of reading newspapers or comments relating to the triggering topic online. 98% of the time it is about someone who has done something significantly more serious than my specific case and when they were in their 30s or 40s. My brain would always seem to focus on the articles which are the most condemning and emotional about the subject or the most extreme and violent viewpoints on forums such as reddit, even though I am ultimately feeling guilty about something I NEVER DID when I react emotionally to the comments. Whilst what the topic of obsession about in my case IS serious, I always agree and side with with the most irrational, angry and violent comments that are made online.

I struggle to understand why I subconsciously try and self flagellate myself despite being told to move on and not think about it. My mind keeps telling me that I have to feel that way because that is what other people in my imagination will say I deserve to feel. When I do get useful advice or people who frame the subject of my rumination as merely something incredibly stupid that I did when I was a lot younger, my mind seems to disregard it or think that these viewpoints are coming from people who have sinister motives. My thinking has blown it up to being morally heinous in the extreme like murder or extreme t*rture. I have intrusive thoughts about that moment, and they appear whenever I feel happy which tell me that I am evil and need to feel bad. It comes with this urge that I have to confess all my misdeeds to everyone I meet and scream it loudly on the rooftops and imaginations of worst case scenarios of how others will react if I did so.

Then there's the mental reviews where I analyse every intricate detail of every single memory when these intrusive thoughts come up to try and get certainty over what happened. It has replayed in my mind 1000s of times and I still don't have the perfect answer for it. If someone gives me advice, it finds something else to attack me with. It comes with reassurance seeking, obsessive googling and spending hours trying to calm the distress that this causes.

How do people who have these symptoms manage it, because I have tried many things like using logic to quell these thoughts but that hasn't worked.