How do I stop thinking about someone who is not going to be in my life?

You need to (a) accept that they are not going to be a part of your life, (I suspect you haven’t yet), and (b) focus on something else.

If you focus on what you can’t have, you will just feel more of your ‘un-blessedness’ so you need to focus on your blessings.

Why not try some mindfulness work?

You cannot stop loving someone even though it is unrequited. If it is just lust, staying away will help. If you love the other person truly, then that love will never fade. Just accept that she/he doesn’t want you in her/his life anymore and stay away from that person.

May be time and distance will help both of you to realize what you both mean to each other. Till then stay away, otherwise you will be hurt so much. Time doesn’t heal anything. It just teaches you to live with that pain.

True love will never go away and you cannot forget that person. But remember that if things are to happen, they will realize someday. I am still crying as I write this one, because I know how much it hurts to be ignored by the one who means a lot to me. Save yourself from that pain.

And don't tell me he /she is the only one.

He / she is not God.

Trust me on this