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

Talk to them. Find out if they do the same about you. Even if they dont, atleast you wont regret not giving it a chance in a hope that it might work for both of you. Say it before its too late. But only say it when youre sure you want to

Do something that makes you happy to get your mind off of them, Take up a hobby or go further with one you already have, start something you've always wanted to do, get a pet. Soon you will be too busy being amazingly happy to think about them

If the thought of him or her is putting yourself down you need to realise that its time for you to be happy again. Understand that you can either be locked in the past dwelling on what you could have been or move on a focus on what they have taught you.

In all Honesty in my opinion the best thing you can do is going and talking to the person in question. Tell them what you are thinking and feeling, they may feel the same way but I am sure if they are a good friend and someone you have known for a while they will appreciate your honesty. If it is a bit more difficult, maybe you keep thinking about someone who has hurt you in someway then talk it out with someone else. Never feel pressured to do so, take your time and say things carefully and with a clear mind. A motto a like to live by is: "What's the worst that can happen?"

try and do something else like watching a movie, reading a book, or going for a walk. do something that will help you get your mind off them.

Keep yourself busy all time. Get yourself so much tired by being busy here n there that as soon as you hit the bed you sleep and have no thoughts of him or her

Take a walk, start a conversation with a friend. Go to social events to occupy your mind. Just do something and don't allow yourself much time to think

Take a deep breath and understand that the heart lets go of things long after we think we're over it. It is a long process and if the relationship was very dear to you then it will become much harder to finally let go. But most importantly, realise that everything takes time and sometimes we won't get things right just by telling ourselves to not do something. (Like thinking of him/her)

Based off personal experience, the only way to stop the thinking is to tell them how you feel. Don't think of the negatives or the positives about telling them just make it your one priority to bring it up as: I can't get my mind off of you... (Then if you want, add the way you feel)

Always follow your heart in terms of love. Of course, it depends what the situation is, but listen to what your heart tells you. Intuition says a lot about it too.