I believe one can’t ‘get rid of’ negative, or even positive, thoughts. We cannot control thoughts, they come uninvited and once arrived stick around until they leave of their own accord. They let go of us, not the other way round. The best way, in my experience, to enable this to happen to negative thoughts, about ones past or anything else is to subject them to what Byron Katie* calls “inquiry”. Its a simple but very powerful technique that anyone can learn to do and consists of 4 questions one asks oneself about a specific thought that is causing you unhappiness. I would encourage you to invest in a book written by Byron Katie called Loving What Is. It saved my sanity when I was in what I thought was a hopeless and dark period of my life. May it do the same for you:)