So,we broke up,he's dating my "bff" and he's a jerk,why?

Well,again,i'm 13,me and my bf broke up a few weeks ago,due to all we did was argue,and a day later,i found out he cheated on me weeks before with my so-called best friend. Now,it's been about 2 weeks,and then,i learn that 2 hours after breaking up with each other,he asked my "bff" out,so,now their dating,and whenever i text him now,he is a toatal a$$hole! he calls me *****,slut,even if i havent done anytihng to him,and my "bff" is just no frickin help..so,i though it wouldnt bother me as much..but it does,i was his 1st kiss,and all that..so..why is he being such an ***?