Me and my girlfriend have both?

cheated on each other. i did it twice and shes done it 3 times. every time shes cheated, she says its because of the drugs that she happens to be doing at that time. ive asked her to stop doing drugs. partially for that exact reason. this last time she cheated it was a with her drug dealer and his girl. again she says she was so messed up on drugs she didnt know what she was doing. i dont know how to make her stop doing drugs. and fully commit to me. im not even mad that she cheated im just extremly hurt. and then im pissed about the drugs because she promisd she would stop. i dont believe the saying "once a cheater always a cheater" because people do make mistakes and if they want to change, they will. so i guess my question is... how can i make her want to change. i dont want to break up with her but this constant heartache is killing me. the cheating, the drugs. ....serious advice please. and keep in mind that i dont want to break up with her.