This will probably sound too simplistic to be of any help to you but give it a try anyway and you'll be surprised at how effective it is.
Write down and memorize 10 positive affirmations and when you notice that you're caught up in unwanted thoughts, repeat your affirmations over and over silently or out loud, until the unwanted thoughts you are having, are crowded out of your mind.
At first the unwanted thoughts will creep back as your repeat your affirmations but if you gently push them aside and continue with your "mantra", you'll find that in time, you will have overcome the bad habit of letting your thoughts snowball as if they have a will of their own. You will be able to switch channels in your mind, any time you want which is a useful tool to possess.
You need to have a "head full of peace" at bed time, so instead of mulling the days events over in your mind when you lay your head on your pillow, simply repeat your affirmations over and over. If you are like me, you'll doze off to sleep instead of rehashing things that can be dealt with at a more appropriate time.
- I am calm and at peace
- I am in control of my thoughts and emotions
- I choose to dwell on peaceful and uplifting thoughts
- My life is not an emergency
- I am a strong and capable person
- I live one day at a time
- My body is relaxed and tranquil
- I do the best I can each day
- I am happy and healthy
- My mind is at peace
Your affirmations should be relevant to your life, be said in the present tense and spoken as if they are so, even if they aren't necessarily true at the moment.