how long does ppd last?

My daughter is 9 months old. I have good days and bad days.. but most days I end up feeling like a failure. A failure as a mother, as a wife, as a friend..as a human being in general. 
I also have a 5 year old daughter. I only work 2 days a week, and by the time we pay for daycare, it hardly pays for it to be worth it for the two days. 
I feel like I cost our family more money than I'm worth. I feel like I'm more of a burden than a contributing mother and wife.  
I feel like my husband deserves a better wife and my girls deserve a better mother.