OP you have my total sympathy. I had PND with my second. This was 30 years ago and PND was not really recognised. It was made worse by the fact that my (childless - but desperate for a baby) sis was living with us at the time. My eldest was very jealous, and, stupidly, I let sis look after baby girl, and I had elder one. (apart from breastfeeding that is). After sis left (when she was 5 months), and all became horrendous. and got worse and worse and worse. (not with the DCs, but for me personally). Eventually, a friend recognised the PND and I started to come out of it...... and up and up (hardest thing I ever did, getting out of PND on my own - like crawling up a mineshaft!)
She was very into FULL BLOWN tantrums! But I did realise that she was so out of control that she was frightening herself. On those occasions, I picked her up and forcibly cuddled and rocked her until her fighting, and then sobbing subsided. It worked!
But poor daughter - I still found her hard to cope with. Who knows? Maybe it was (is) her personality? Maybe it's all my fault (and my PND)? She continued to be very difficult throughout her childhood, and, more particularly her teenage years!
But we've fought and talked and fought and talked........... She is now 30, and absolutely lovely! She regularly rings me and I her and I just love talking to her, and, rather flatteringly, she regularly asks me for my for advice. She lives in New Zealand, and I often wonder if she stayed there (after her Gap Yearn12 years ago) to get away from me!
Ashamed I really DO understand how you are feeling, but please don't make my mistakes! I think (given my time over again) I would use 'positive discrimination' (ie make him your favourite) to compensate for my appalling lack of mothering skills with my daughter. I really suggest you do it with your son. Honestly, with children, you get out what you put in!!!
And you really don't want your son to be posting on 'Statley Homes' in 15 years' time - saying that he is the scapegoat, now do you?