((hugs))
I feel for you sweetie!
I do think it is possible for it to go from PPD into just fullblown depression. With the birth of my first came this overwhelming sadness (sleeplessness, crying spells, 'bad thoughts' you name it!). I made the mistake of asking anyone I could for help, including my dr. All of which just patted my hand and said get over it. My husband and I both were very young..and when I started complaining about the symptoms and saying that what was going on inmy head wasnt right and that I needed professional help, my husband said he wasnt going to incure more medical bills as he was overwhelmed by the hospital bills form the birth of our firstborn. I write it up to immaturity on his part...and mine for not getting the help I needed then.
That feeling didnt go away... and 10+ years later and 2 more kids... I finally BROKE! I had a breakdown and was diagnosed with depression. The dr did the usual history and did seemt ot hnk that the untreated PPD may have easily led to full blown depression.
It is my sincere hope that you seek the help you need. The hell I lived is nothing I would wish on my worst enemy. You deserve better than that. Your family deserves better than that! ((hugs))