Negativity is like burning plastic: while destroying itself, it annoys everyone with its stench.

What starts out as an intrusive thought can turn into an overwhelming concept if we "feed" it with more negative thinking. From the book: Removing Your Shame Label.

If I am not persistent with my desire to think about other things, and consciously initiate new circuits of thought, then those uninvited loops can generate new strength and begin monopolizing my mind again. To counter their activities, I keep a handy list of three things available for me to turn my consciousness toward when I am in a state of need: 1) I remember something I find fascinating that I would like to ponder more deeply, 2) I think about something that brings me terrific joy, or 3) I think about something I would like to do.

weeding out the harmful influences should become the norm not the exception.

Negative thinking is associated with low self-esteem. A person who is low in self esteem displays his disappointment in all aspect of lie. He is victim of anxiety and emotional turmoil. He lacks social skills and self confidence. Low self esteem is the source of almost every problem in our life. It is important that we identify the self limiting beliefs that are created due to it, if we want to if we want to improve our life.

The key to happiness - or that even more desired thing, calmness - lies not in always thinking happy thoughts. No. That is impossible. No mind on earth with any kind of intelligence could spend a lifetime enjoying only happy thoughts. They key is in accepting your thoughts, all of them, even the bad ones. Accept thoughts, but don't become them.
Understand, for instance, that having a sad thought, even having a continual succession of sad thoughts, is not the same as being a sad person.