Depressed Need Advice..

So My ex best friend now started dating a boy I use to talk to and this isn't the first time she's dated someone I talked to smh but she chose him over me and I feel so alone now we were best friends for more then 3 years and she's known to be a ... don't want to sat it but I never judged her . Basically I don't know what to do I have no friends and I'm not making any in school cause everyone is talking behind everyone's back and this is what I've been going through for 3 weeks now I need some type of help/advice