It starts with Presence. When we spend time thinking of the past and future, we cause our own suffering. Nobody has control over the future so there is nothing constructive about what-if'ing. The past is now just a story, and identifying with the past, especially when those thoughts make you feel bad, is useless.

There is a spiritual teacher named Eckhart Tolle who really helped me shift my negative thinking. Listen to this audio and the Part 2 that follows:

And this one:

In fact, just listen to a bunch of his videos on youtube to get a sense of what he teaches. He wrote the book The Power of Now. I have found his teachings so transformative that I simply don't engage in the kind of negative thinking that leads to use being negative to the point that our friends get annoyed with us. Give it a try. You might be surprised!