Love is its own kind of drug - mainly endorphins. So you can literally go through physical & emotional withdrawal. Finding other (legal, safe) ways to boost your endorphin levels can help a lot.
(What do YOU like to do, that doesn't involve being with him? Meditate, read, sports, window shopping, music, write, movies, clubs, etc.)
Positive reinforcement helps emotionally & mentally.
I know I didn't want to hear this as a teenager, but - talk to your mom/female adult parental-type figure. They need to know you hurt so they can help support you.
And give it time.