Sorry to hear that. Sometimes you can't stop your feelings for a guy. Well you have two choices. One you could keep believing and wait a while until he gets over the other girl. Or you could move on and get over him. Remember that's no matter what choice you make, make sure to listen to your heart and never give up. Good Luck :) <3
If you are under the age of say 18 then my help is this: You are not in love with this guy you are in love with the idea of being in love. IF you are over the age of say 21 then you are in love with the wrong guy obviously. Love is not to be taken without giving it right back.