Recently I remembered a terrible thing from a long time ago and now I can't stop thinking about what happened or getting worked up over it.
How miserable for you! Did you try more than once to stop thinking about what happened?
Yes, I've tried to stop thinking about it in the past and on some occasions I could.
If you keep on ruminating on events on the past and can't move on from them, it would be a good idea to reach out to a mental health professional. Therapy can help your resolve the issues that are keeping you worked up and you will once again be able to live life without intrusive thoughts affecting your mental health.
I'm fine and this is small shit.
You said you can't stop thinking about it, so it's not really fine is it? There's nothing wrong with talking it over with someone who is trained to help. They will help you deal with the memory and then you can put it behind you.