[jennyjuniper]

As an alcoholic myself (dry for over 20 years) I'm about to offer some advice which may sound cruel and heartless to some. Alcoholics are totally selfish and self absorbed when they are drinking and while people mean well when they say help your mum to stop drinking, only she can do that and it's only her responsibility not yours.
Help for children and family of alcoholics can be had from AA. They can support you and give you good advice.
If you want to go to the States then go, but live by yourself then you don't have to put up with your dads girlfriend.
Good luck and do what's best for you not anyone else.