Most Helpful Guy
It's perfectly normal. This happens to just about everyone at some point.
The best way to fix this is to minimize your contact with that boy--don't be rude, but just stop talking to him/being around him. You may want to speak to him first to let him know that you still have some feelings and that it would be best if you didn't talk for a while so that you can get over him, that way his feelings don't get hurt or ruin your friendship. But then you need to stop talking to him until your feelings are gone. And when you have thoughts of him, you have to change what you are thinking about--you can do that.
Next, you need to start talking to other boys. Doesn't mean you have to date them, just get used to the idea that there several billion of them on the planet, so you don't have to worry about liking just one :) And make sure you have fun with a bunch of female friends too.
And most important: just remember friends/dating is all about fun. Don't take it too seriously. Hope that helps ;)