So you think about a particular guy, Does it make you feel nice.
I have been in same situation. I used to think i liked her. Life can be strange , we became friendly and than all my hopes from that relationship crashed.
Reason was that i had some how without realising I had made a mental image of that person. My point is that whatever we do in life is for pleasure and sometimes the whole idea of thinking about a person, activity gives us more pleasure .
When i started photography i started reading a lot on net abut photography , saving lots of web pages. I realized more than photography the idea of searching info on photography gave me pleasure.
I can recommend you to do meditation.
I have started doing recently and it is great.
Simply close your eyes and when you start thinking about him, just try control the feelings, by controlling i mean lets suppose he is a funny guy and something you remember that he said and that makes you laugh, when remembering him don't flow with emotion.
Keep remending yourself he is a nice guy but he is not worth my peace, he is not worth that i start thinking about him every moment.