Negative thoughts have always plagued me, but I've learned to combat them.
First, be aware that you are having these thoughts. Ask yourself “What is it about this situation that is upsetting me?”
Second, challenge these thoughts. Just because you think something, doesn't make it true.
Third, put your thoughts in perspective. Are things are bad as you make them out to be in your thoughts? what is the worst that can happen? What can you do to get through the situation as best as possible.
Last, what are the more helpful thoughts. In your case, what can you do to further your job search.