How do you cure over active thinking, overthinking, or paranoia? how can you get over things that you know are just in your head?

See your doctor: although most people would probably say that you need to see a mental health professional right off the bat I always like to refer people first to their primary doctors to get a good physical exam and some basic labs and chemistries. Conditions such as hyperthyroidism and poor sleep hygiene can often be significant contributing factors to the emotions and behaviors you are describing.