How do i stop perseveration on the same thought over and over and over again?

No short answer: If this has been a long term problem and is interfering with your daily life then i would recommend a psychologist and therapy.

Obsessional thought: Sounds like you 're describing an obsessional thought or worry. If this is taking up a large part of your day, causing you distress, and affecting your functioning, you may have ocd. Cognitive behavioral therapy usually the first treatment to try, usually medication ( typically the ssri's plus clomipramine) plus cbt give the best results. See a therapist trained in cbt and a psychiatrist for meds!