How can I approach my mother? please read details?

I am a new mom, and my relationship with my mom has always been very rocky. I can pinpoint my recent problems with my mom to the day after my daughter was born. I was in the hospital and i literally begged her to stay with me until my partner came back. I am only 20 and she is my first born and i just really didn't want to be left alone but she left me because there was something good on tv. This is her first grandchild we are talking about here... My mother is an alcoholic, and has been addicted to heavy smoking and cronic pot use since she was 13. She can be mentally slow sometimes but is often purposely hurtful and stubborn. we have been fighting a lot about her life choices because it is something that i am choosing not to expose my daughter to. where i wasn't given the choice, she will have the healthier choice made for her until she is old enough to think for herself. long story short, i have been trying over and over to write a letter to my mom about her behaviors that have been depressing me so much lately. she lives two blocks away, and hasn't called or visited in 3 weeks when i am home alone on maternity leave and she is currently unemployed. it hurts that it seems she doesn't care. i feel i have to say something but no matter what i write it sounds like i am calling her a failure as a mother and grandmother. as you can tell i am very confused right now. i can't sleep at night. i don't know what to say that gets my point across without making her so mad she'll never speak to me again. the situation is delicate. HELP!!! so lost.