Like I said, I am not an expert or anything like it, on this matter. But the way I do it is by simply thinking about where did the thought come from. Who was the source? A person? A TV? When did I start thinking about this? Usually, I get the answer right away.
For example, I am a basketball player and I had many problems with injuries. Even though I am good now, but sometimes my mind starts thinking again about the injuries when somebody on the team gets's injured or starts talking about them. Like I said in the text it can from certain negative situations in life or simple negative talk. Like a talk about injuries. Those thoughts came to me from someone. In this case my teammate. My teammate was a trigger which brought back all my negative memories about injuries I had in the past.
I can't, of course, control when someone gets injured or starts talking about injuries. It is part of our profession. But I can move away from the talk about it, or I can give advice to someone injured with all the experience I have about it. In that way turning negative into positive. Helping someone get better faster.
It is all about turning those negative situations into positives.
But, like I said this is just me expressing my thoughts about the subject from my personal experience. Not an expert in any way. :)
I hope it helps. :)