If you tell her and break up what's the point of having told her. If you stay together for along time every time you get into a fight she will throw it in your face or blame your "temper" on it. She'll spread rumors about you when you break up. If you get married, then get divorced... she'll use your drug use to win custody and have you tested for illegal drugs. Or have a private eye follow you, get pictures, and use it to extort everything out of you that you ever had.
Or she'll have her brother call your ISP saying he lost his password, she'll know all your info and SS#, then she'll see all the sites like this one, find your posts about it, and submit it to Court.
Sounds like a good time.
As someone in the matrimonial law field, I can guarantee to you that this stuff can and does happen all the time.
You must be very protective of your privacy at any and all times. He who lets his guard down WILL live to regret it.