How do i let him go?

i was only wit him 4 5moonths but i really did fall HARD its been over 4 a month but i am just now excepting the fact that it is over it would be easier but i have to see him every day at school. just sittin in the same room with him puts me in such a bad mood i used to be such a sweet person but my temper has been gettin the best of me i dont sleep i really miss him how do i let go of sumthin thats no longer there