I've found throughout my life that positive affirmations and visualization are effective tools to counter negative self-talk. I would even recommend a person passionately say to themselves, "STOP, CANCEL" when negative thoughts arise, and then go to the positive affirmation. In someone is in a crisis situation or really depressed, I've found that reading Bible verses can be helpful, especially Psalms, for positive reinforcement and perspective. I also tell people who think negative that's it's just as easy to think positive instead. But like forming any new habit, it takes persistence,