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

Tell them how you feel about them, talk to them, hug them, make them feel special about themselves. If you don't tell them how you feel, they'll never know. Well, unless they are a mind reader.

If you want to stop thinking about him. I would recommend taking yourself out on date. Find out what it is you get from your relationship from him and start doing the things he used to do for you for yourself :)

Keeping busy is what always helped me in this situation. Work on yourself, have fun with friends. Whatever you think is going to improve you and keep your mind off of things. :)

Try to just focus on yourself and figure out why things did or did not work out and work on what you can change with you.

There's two ways to approach this in my opinion. The first way is that if you really want to get back with them, you should just at least try. If you caused the breakup, it would be right to apologize to them, and if you want to get back with them, talk to them about it. Otherwise, if you want to stop thinking about them, you should occupy yourself with other things. This could be interacting with friends/family, or just doing something you enjoy.

It is ok to think and long for someone else. To make it easier and much more healthy, focus on yourself and exercise awareness, acceptance, and commitment of yourself and of where you are. Doing this brings back focus to you, and makes you important to yourself. I don't think we ever forget or stop thinking about others, no matter what the context, we just need to think of ourselves as well.

When I am feeling anxious in my relationship, it helps for me to go on a walk, or to get out onto the sun for fresh air.

Be patient with yourself. It will get easier and you will begin to think about him less. Go out and have some fun and do something that makes you happy.

There's a lot of emotions which can be present when you think about a certain person. Reflection and understanding the situation can help in healing or moving on with your personal life. It's never a good idea to ignore the pressure you may be feeling with moving on. Even so, it can be uncomfortable remembering why you parted with him or her. But the most important thing is accepting and coming to terms with what has happened. This journey can take a long time and may feel unsatisfying, but little things like having more self-love and appreciating the things you have done well in life can help.