The answer to stop going over painful memories from the past or fixating on worse case scenarios in the future.. is just to allow yourself to live in the present moment. It will give you a break from the constant worry and unproductive thought process and allow you to relax and open the mind in the now. Take a few minutes out of your inner turmoil and allow yourself to focuss on something small. Sitting, lying down, breathing, eating etc. Your body and mind will appreciate something new.