Excessive and extreme thinking loops.

I tend to think too much about anything which may not be very important and get caught into the loop which hampers other activities. Could not focus on studies.Always  trying to justify that in case  something happens it was not due to my fault.Always have a feeling that my decision would not turn out to be correct and its also because it has  happened in the past that things hadn't turned the way i wanted.Always consider myself guilty of not being able to give my 100%.Always think about something exteme happening and then my mind sets into the motion of what then i would do.Before any exam i do prepare well but as i approach it before 1 or 2 days i get too distracted and could not focus well.And also after doing something i tend to think whether i did right or wrong and then the whole loop starts .How could i come out of it? Plz help.it seriously hampers my studies.