I ****** up really bad and need advice?

I've been feeling depressed and suicidal for a while now but even though I hang with the popular kids, I still feel like a loner. They said that drugs helped so I started smoking pot and now I can't stop and last night I was stoned and I had sex with my 19 year old friend and I don't know what to do and I have a gf who loves me but I cheated on her and I want to tell her but I don't know how.