What do you do if you like a boy but he likes another girl?

Don't try to keep a hold of him. Give him a little room and don't try to put your self out just to get his attention again. Yes things may feel all jumbled up inside you and your hurting but if he really actually likes her instead of you then he would usually leave you. But if you give him a little space and show him you still care for him (but not in an obsessive way) and he still feels something for you, then he'll stay with you.

If you're close enough with him to talk to him about it then do so but if he doesn't feel comfortable talking about it be sure to not push him and then end up loosing him whether he actually likes this girl or not.