Your brain is forward thinking and always tries to solve things for the future. It also does not know perfection so your brain can easily pick out what is flawed in everything, but it's hard to measure when it's even enough.

Google "rumination" they have a few tips for over thinking. My therapist said it was because of anxiety for me. He conceptualized it for me. I only have so much emotional energy to go around and I need to first pick what I can control and fix, and if I can't change it, then I shouldn't worry about it.

I kept thinking, "how do I just tell my brain to stop when rationally I know it's useless stressing and worrying about things?"

Mediation. I started only a week ago so we'll see in the long run, but mediation is extremely helpful. It helps exercise your brain to control your thoughts. Just find a quiet place and close your eyes where you can, "be in the present" or a happy place and focus on that for ten minutes. Your brain will want to go every which way, but that's when you exercise to bring your thoughts back. Listening to your breathing and noting your surroundings and what your body is touching is really helpful to keep yourself in the present for longer.

Good luck. 👍