If you still overthink, even when you know that it’s not the best thing to do, at least try not to fight with it. Welcome all the thoughts and flashbacks. The more you resist, the more difficult it will be to get rid of the thoughts.
One thing that can be helpful is some DIY projects or crafts. When you focus on something, especially if you want to have a great result, you will think only about what you are creating at the moment.
This way you can rest your mind, at least for a little while. I know that not everyone likes sports, so crafts are a good idea for people who don’t like physical activities. Also, I would like to suggest meditation, but meditation is not something that is easy. It requires a lot of practicing, but when you are finally able to do it, it’s the best thing ever to clean your mind. At the same time it’s good to do breathing meditation.
While you are meditating, you can also do active breathing. This is breathing that goes higher and higher inside your body. You start from the stomach and each time you breathe higher, and then you start again.
Do this for at least 15 minutes. Focus on your body, and when you breathe focus on doing it right. Don’t give up even if you can’t get successful with it after 50 times. Keep going and try until you are able to do it.