M.L.
I'm just wondering why you say you chose not to become a mother, you said it twice, so it struck a nerve in me. Perhaps you also have resentful feelings towards them and the person who apparently coerced you into birthing and keeping multiple children and this is a major underlying factor into your issues.
PPD typically ranges from birth to one year, and if it's much later, then it's a separate detachment and depression issue.
I would implore you to seek professional counseling. To deal with your attachment issues, your resentment, your emotional withdrawal, which by the way is a form of emotional neglect. The children deserve a mother who loves them and SHOWS them they love them regularly, and you deserve to be happy and comfortable in your life decisions. All can be attained and a happy medium can be had. First, you need to understand that in the long run, even if it was a rocky beginning, you have chosen to be a mother, you could have adopted out, run away, aborted, left the person who was getting you repeatedly pregnant... but you chose to stay so that's something.
Also, perhaps you just need a break. If you need to work outside of the home, or join a mother's group or a hobby group of something you love, you can do that too so you don't always feels bogged down by taking care of little ones all day. It's tiring I know! I sincerely wish you and your family peace, and luck.
Also, here is some expert advice on maintaining anger under control to kids:
http://www.askdrsears.com/html/6/T061800.asp
and using humor to discipline:
http://www.askdrsears.com/html/6/T061400.asp
And this book is called, "Playful Parenting" by Lawrence Cohen and has great advice on teaching you how to reconnecting with your children and teaching them through positive discipline.
http://www.amazon.com/Playful-Parenting-Lawrence-J-Cohen/...