I can't stop thinking about him or her. What should I do?

Accept that it's natural to think about someone who has had a major impact on your life. Then, when you're ready, try to find other people who can fulfill some of the same needs your ex did.

Engage yourself in something you enjoy. Like listening to music or sketching or gardening? Try to throw away all things that remind you of him like gifts he gave.

Get busy. Spend time with family, go out with friends, go walking/jogging. Going out in nature is great.

I underwent the same experience a week ago and what helped me the most was to dive into a productive and mentally conducive hobby I love i.e. reading. Alternately, I also linked the subject to a different subject and would try to immediately switch topics when the former crossed my mind.

Tell him or her how you feel because then all this emotions will build up inside you, The worst thing that could happen is you them not liking you back.

Like rechargable batteries take yourself out of the source of your stress and take some time off for yourself and recharge yourself to become become a better person for you and those that matter to you that bring out the best in you.

Try focusing on a fun activity that can keep you busy. Coloring can help you keep you calm and exercising is good for body.

Just try to stay busy. The busier you are during the day, the less time you have to think about someone

It's hard letting go sometimes, but that's ok. Let those emotions come, but make sure you let them go. Try to think of them as waves and let them wash over you knowing that they will eventually recede. When they pass try to focus on the positives in your life, try to create goals that help you achieve things you are happy about and things you look forward to. Surround yourself with positive people who encourage you and try to remember that you will have days when you slip up but that is ok the more you fight to move past these feelings the happier you're going to become.

It's difficult, I know. But I promise that at some point it will end! You just need to push through it. Do what makes you happy, discover and do new things. It will help you to know who you are without that person.