For me its all about distraction. There are times where I just cant shut my mind up, so thr only way to combat the negative thinking is to ask myself "is this thought helping me?" and then do something. Pick up a book, turn on the tv, come onto the forum, go for a walk....do anything but let your mind have free reign.
It also helps to have the right medication, for my psych keeps telling me its 50% meds and 50% positive thinking, neither can keep you stable on their own depending on what you suffer from. (For me its bipolar).

You can win the fight, and thats what it is a fight. They say bad thoughts are like stray dogs, don't feed them and they'll leave you alone. In other words dont ket yourself sit there thinking. Get active. Do whatever it takes in a positive way to break the thought pattern.

I feel for you in your struggle, but please keep trying. Its not always going to feel like it does now. When you feel like giving up, try to remember why you have struggled not to give up in the past. You can do it.

Hugs