Hi bnknzr and welcome. So you and your gf are about 18 now? She likes to go out to the pub or wherever with her friends and drink, that seems pretty normal really.
She has been kind and understanding about your concerns, can't fault a person for that!
I can absolutely understand your reaction though, your fears. You basically "grew up" with an alcoholic in your presence hurting you every step of the way with her drinking etc etc. Why wouldn't having your current gf going out drinking affect you, it's still all very fresh and I doubt you have really dealt with any of it.
I think seeking out a counsellor or therapist would be a really good idea. This will set you in a better place going forward and in all future relationships. Ask for and get some help, you will feel so much better.
What you are going through really had nothing to do with your current gf at all, she's not doing anything "wrong", your reaction is completely normal at this point, so there is no blame to be had and from what you posted you seem to totally understand that.
If therapy or counselling is unavailable to you, you might try attending some Al-Anon meetings. These meetings are for people who have been affected by friends and/or family that are alcoholics. Lots of wisdom there and tools to help you deal with how alcoholism has affected your life but more importantly perhaps, how to focus back on yourself and your life.