That is an agonizing, horrifying and excruciatingly painful story. Your girlfriend's mother and every human being who violated, or allowed her to be violated, should be in prison; their actions are incomprehensible and reprehensible to say the very, very least.
Have you read the book "The Shack", JD? It may prove helpful to you; I read it twice and found it powerful; in a way, it changed the way I viewed many of the things I could not come to terms with in my life and in this world.
JD, you are adding goodness to the world, to this woman's life, through the support you offer to this woman's life. We can't control the vast majority of the events of the world but we can control what we add to this world. I hope that you can concentrate on, and find comfort, in the positive effect you have on the world.