Just as there are clouds in the sky which come and go, thoughts come and go. Meditation teaches one to observe the thoughts and bring ones attention back to one s breathing. Try to fix a time of the day for thoughts. At that time write out 2-3 of your thoughts. During the day try to catch your thoughts and day out loud to them- Not now, I'll come back to you at 8 pm. Take 2 deep breaths, Count 10-1 backwards slowly and day out to yoursel- Relax, Take it easy. Keep working on it. It takes time

What was the major situation or event which pushed to self doubt yourself? Was it the situation or surround people around you ?!! There is always reason behind every behaviour in the present.  So do consult a psychological Counsellor in Jr city and get a clarity on these issues.

Well getting repetitive thoughts which are not in your control from any very early age in a working professional is strongly in favour of an obsessive compulsive disorder (OCD). Getting bad thoughts about future and self also is a part of OCD. These symptoms usually maintain a waxing and waning course with symptoms worsening during stressful situations. These symptoms you have described can also be a part of depression or other anxiety disorders but are highly unlikely from an early age of 12.

Hi, there are many more details that will be necessary before we can come to a diagnosis, which is very important as far as treatment is concerned. You have negative thoughts, too many thoughts and negative thoughts regarding future and some existing situations. This could be part of Depression/ Dysthymia or Anxiety disorder with Depression or Anxiety disorder. As it is affecting you , I will suggest you consult a Psychiatrist. Once a diagnosis is reached, a treatment plan can be worked out. Mostly you may not require medication. You will benefit greatly from counseling or therapy. In the meantime make some lifestyle changes and changes in your perceptions about yourself and things around you. Appreciate your strengths, focus on the good that is happening in your life. See yourself as a strong person who can definitely get rid of this habit. Hang out with people who make you feel good . Exercise daily, it helps release feel good hormones. Eat small meals regularly, eat healthy. Sleep adequately. Hope this helps, All the best!