Negative thoughts are common to all of us with a prefrontal cortex, they are an adaptive mechanism of survival. Earlier in our evolutionary history this habit of "watching for signs of danger" was quite useful. However, in our society these days there is less need for such a mechanism than in the past. Our mind however does not so easily toss away tool that have served it for eons. This leaves us with with many refer to as the human condition. In fact, speaking generally, humans negative thoughts outway the positive by roughly 4 to 1. Where many run into distress, is in trying to "stop thinking" negative things. This simply is not possible, and the more a person resists against this natural part of themselves, the more and more these thoughts usually grow. Both the response above refer to "letting go". This is indeed good advice. Here are a few suggestions in addition to the good ones already posted.

1) Realize that your thoughts are just that, thoughts. Our brain constructs hundreds of thousands of thoughts per day. They are like clouds that pass across the sky of our mind. Let them pass by, as they do, take a look at them, see what they look like, but dont slow them down as they go by. You can simply say "thats an interesting thought" and move on to the next cloud.

2) Just breathe: give your mind something to focus on. Your breath is always with you and provides many sensations to focus on. Spend a few minutes taking full deep breaths and task your mind to follow these breaths. It will wander off, just guide it gently back to your breath. Deep breathing actually changes your mental chemistry and will affect the way you think.

3) Thoughts rarely reside in the present, they almost always are off worrying over what might come to pass or what has already happened. This isn't very useful since we cant change history or see into the future. So when you notice yourself caught up in thoughts that aren't about this moment, take a moment and notice 5 things around you. These could be sights, sounds, tastes, smells or other sensations. Being present to what is happening right now, is an excellent way to stop mental suffering.

Anyway just a few thoughts, about thoughts.
I hope you find some peace soon.
~Respectfully,
Tantric