A couple of things. I can clearly see this now in my own situation. My daughters are now 8 and 14. The worst mistake I ever made was staying. My 14 year old has resentment you would not even believe. What I am seeing now is not only do they resent him and what his behavior does to our family, they also resent me.
I am angry and resentful. I am betting when you have to take him on his little drug runs you are too. Do you think that does not wear off on your daughter? Unless something changes in your home she will grow up with this anger and resentment.
I encourage you to get therapy and to go to meetings for support for you. You deserve more than this, but ultimately your child is #1. You brought the child into this world now every decision has to be what is best for that child. Is having a father who has a "little" drug problem productive to her? I doubt that very much.
I am not trying to be harsh with you, just trying to help you see the situation in a clear manner. Why should you and your child have nothing while you give up your extra income for drugs? It really is unacceptable. I am sure you work hard. Think about how many hours you are putting in so he can go on a drug run.
Good Luck and God Bless.