What do I do?????

Somebody tell me please, how do I explain to my 7 yr old daughter why her father chose a life style of drugs and alcohol instead of her? She is so smart, and she is asking more and more questions about him, and what happened between us. She has blocked out majority of what happened with our day to day life with her dad, so she is trying to piece it back together. I've tried sheltering her from what happened but that just didn't feel right. I am honest as I can be with her, but I am running out of answers.... I don't know what else to do..... My baby is growing into a little lady with out a dad. My dad is her male influence, and he is gone working all the time. Like being a single parent isn't hard enough!! He just walked away from her!!! Didn't even put up a fight for her, I told him to stay away if he can't stay sober!! He never even tried.... never tried.... He just disappeared. So she has no Dad. How can I be everything she needs? How do I do this? How? I feel so over whelmed, I just want to scream. I'm sick to my stomach everyday now. I can't sleep, can't eat. She is needing the one thing I can't give her!! She is hurting so much, and I can't take away her pain. All I can do is help her deal with it. A 7 yr old should not have to deal with this!! I wonder some days if I am strong enough to get her through this. I honestly don't know where it comes from sometimes. I am so exhausted, what a time for a magic wand. We are both learning just how many ways his abuse is still hurting us, and he is not even here anymore!!!