meth is destroying my marriage

[distraught and heartbroke]

I am a man in his 30's and years ago I had a young daughter that I had custody of that suffered brain damage at the hands of a drug user (my gf at the time), I do not know if it was by meth but I had witnessed her using this as well. She is in jail serving time for hurting my daughter and manufacturing meth. Fast forward to present. I am now married for less than a year to a woman from the same town that uses pills and now meth! I love her with everything in me and I want to help her but she transforms into something evil while on the stuff. She is physically , verbally, and emotionally abusive to me. My daughter has told her grandparents on two occasions that my wife has woke her up during the night while staying with us and said things to her and once threw a foam ball at her. I have never asked her about this, my wife, that is, because she would say that my daughter is a liar and that is why she doesn't want her around. She says to have her with us is scary. She does not like my special needs daughter or my two from my first marriage 16 years earlier and she says things that I will not repeat but that I would not say to my worst enemy about them. To give you one example, two days before this new year I went and got my daughter to spend time with during the holidays. I have a side job in the afternoons and was scheduled to work, I had to call in because my wife refused to watch her and keep coming into the living room from our bedroom whispering insults about my daughter under her breath and slamming the door each time she went back into the bedroom. We had a crappy new year because she was clearly on meth and then we had sex on new years day, but as always it was empty sex with no feelings involved-I really hate that but I love her and I do not deny her sex because I feel that if she knows that I am willing to make love to her that maybe she will see just how much she actually means to me and, for fear she may go somewhere else. I have never used any kind of drug unless you count goody powders for headache and I have pleaded with her to stop. I have left numerous times in 2012 for a day to a week and each time we get back together. She is now up the road at her mom's house about a 10 minute walk up the road and has been there for the past four days!! Her mom uses as well and misery loves company. Both of my wife's grandparents passed away last year within a few months of each other and this is her reason for beginning to use again but for six months now!! I am so mad and hurt! But in the same I hate how the person that never said a bad word about my daughter, almost killed her and I am willing to live with someone that talks as if they would finish the job if given half the chance. I do not have custody of my daughter but I want to get her in June of this year I keep saying that if my wife is not on board that I will leave but I already know that she willl not be...why do I keep picking these kind of women that end up stealing my heart but won't stay straight for the sake of our relationship??? I have packed all of my things and they are in the car. I know that when I leave tomorrow to go to work that she will come home and I am leaving a note saying, "me or the drugs, get help or it's divorce!" If I leave her and my 18 year old step daughter will lose everything because neither works, I would feel guilty but I cannot live like this anymore. It's not fair to me to work, clean, do laundry, pay every bill, maintain the house, cook my dinners, pack my lunches, and be put down because I do not make enough income to pay our bills without using the grace periods. At least they are getting paid! She says she loves me and when she is straight she is the woman I fell in love with. I need advice any is welcome good or bad..I am desperate.