The best way I found to avoid negative thoughts is to find the things that trigger them and avoid them or cut them out of my life. Lifestyle changes are needed and can be difficult or impossible to accomplish, but even small changes can make a huge difference.

Switching from 4 on 4 off 12 hour shifts to a steady 5 days a week 8 hour shift, with 12 hour weekend shifts every 3rd weekend made a world of difference. Luckily, my employer made this change.