I want to stop thinking about my ex. What should I do?

Try exercising or picking up a new hobby. It's an awesome stress reliever. Instead of thinking about your ex your focus will be on your strength/body or new hobby.

Divert your attention elsewhere. Have fun with your friends, learn new things, go on trips, enjoy your time with your family. Diverting your attention to something else will make it easier to forget him/her :)

Go the do something and take action for yourself and that way you can immerse all your feelings and concentrate and leave him behind. Just push forward and do your work to dostract you from thinking too much about him, don't beat yourself up about someone that's no longer a part of your present.

Get yourself a new fun hobby! It can represent a change in your life and it should get your mind off things.

Focus on other aspects of life, try to move on. You were vulnerable at a point become stronger emotionally and it should work wonders

I remember the feeling well of being able to think of nothing but my ex for what seems like forever. Keeping busy, doing things you love, spending time with friends, reading books all seem to help, but they won't stop the thoughts completely. I'd recommend accepting the thoughts as they come until they occur less and less frequently and one day you realize you haven't thought about your ex in months. It takes time and patience, but it'll happen.