Hi OddBob,
I had a horrible abusive childhood and got out of the house asap but got a job as a waitress and house painting and put myself through college. Life is a series of choices that determine our destiny and while your girlfriend has told you a sad tale of woe as to why she has been using drugs heavily for many years and continues to use drugs it has been her choice to do so.
I have helped staged some interventions with families and strongly suggest that you hire a professional to talk with all of you privately to discuss the situation. While interventions can be successful IF the addict is ready to break up with drugs and alcohol for good - this does not describe your girlfriend at all. She sounds like she has zero desire to alter her lifestyle but instead, hopes to bring you around to agreeing to getting high with her!
My XA got several girls before me strung out on crack cocaine so they could help finance his drug habit. He was known as a predator at AA and NA meetings who 13 stepped women away from recovery. Of course I didn't know this for a long time and only found out his true history after we had split up!
Back to interventions... it is very important that there is unity and clear plan for plan A and plan B... acceptance and recovery or continued addiction. Addicts do not hear or process information very well when it threatens their addictions so framing statements and boundaries should be done carefully.
It is important that any boundaries set are followed through for your own peace and serenity. Normally interventions are a plea that the addict get help immediately or ties are severed until they get into recovery. Don't be surprised if your girlfriend refuses to seek help and finding a bed for her without her doing an intake interview is not easy. No one wants to deal with an addict that isn't ready to quit...its like putting lipstick on a pig as it just makes the pig really, really mad and doesn't do that much for their looks either.
Good luck