How can I stop overthinking so much about the people who hurt me? And what is this disorder?

Forgive them, then get them out of your life. Once you have forgiven them, there is no reason to think about them anymore. You are thinking about things you could have said or done, but all that does is put you back in the obsessive loop you have programmed your brain for. Now you have created an obsession in your life, and have to figure out how to break the obsession. Cognitive Behavior Therapy, Retrain the Brain, or something along those lines may be of help to you, or you may require professional help. If you want to try going it by yourself, I would recommend something like Cognitive Behavior Therapy for Dummies, which you can find at your local library or used.
You made this problem, and I believe you can unmake it. BUT, it will take effort...
Best of luck,
Ivery

Kealila said:

I think of them every single day. Yet I have not been in contact with them for months or years. They are all different people. I get very angry and frustrated every day. And I think about all I could have said or done to defend myself. I think about explaining to them how they behaved wrongly even though I did that already but I feel they did not really get it. I know I must let go. I am just not able to. I am literally wired to think about that and feel anger and frustration every day. It is exhausting and so distracting. And it has been going on for years for certain things.
I know I must accept it happened and let go but I dont know how to concretely do that.

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It's normal, it's just rumination. It's a normal response to certain types of traumas, but it is a normal response.