My personal belief is that we can’t control every thought, but we can make sure we’re in a positive feedback loop instead of a negative one. For example, when I am getting negative, I’ve learned to take a step back and try to figure out why. If I can’t get myself in a better mood, I go to bed early and generally wake up happier. I also try to structure my days to make time for the things that make me happier (exercise, eating well, friends, family, hobbies) and cut out things that make me feel more negative (social media, news, sugar, etc). Not perfect, but it helps.