One of the ways you can train your brain to be more positive is a practice that helps to rewire your neural pathways. Neural pathways are comprised of neurons connected by dendrites, they are created in the brain based on our habits and behaviors. As you take up new activities you are training your brain to create new neural pathways. The pathways get stronger with repetition until that behavior is the new normal.
Try writing about 3 things you are grateful for & do that for 21 days in a row., be mindful of spending a few minutes doing this. At the end of that time your brain starts to retain a pattern of scanning the world for the positive and not the negative. Mind hacker & author John Hargrave teaches how to take charge of the mind & improve mental habits, getting rid of negative thoughts & anxiety. His book Mind Hacking gives a 21 day programme, but you could try this yourself as a good way to start with just a notebook & pen.
Other really useful books that deal with this subject are:
Atomic Habits by James Clear
“Atomic Habits is a practical guide on how to create good habits, break bad ones, and get 1 percent better every day. I do not believe you will find a more actionable book on the subject of habits and improvement. James Clear, one of the world’s leading experts on habit formation, is known for his ability to distill complex topics into simple behaviors that can be easily applied to daily life and work. Here, he draws on the most proven ideas from biology, psychology, and neuroscience to create an easy-to-understand guide for making good habits inevitable and bad habits impossible.”
Tiny Habits by BJ Fogg
“I’ve researched human behavior for over 20 years at Stanford University. I’ve learned that only 3 things will change your behavior in the long term.
Option A: Have an epiphany
Option B: Change your environment
Option C: Take baby steps
Creating an epiphany is difficult. So rule out Option A unless you have magical powers—I don’t.
On the other hand, Option B and Option C are practical. And they can lead to lasting change if you follow the right program.”