My HV used to say to me "You don't have to be a perfect mother, just a good enough mum" but I couldn't see it. Not until I came out the other side of PND and compared my mothering of dd with how I had been with ds at the same age. You always want to feel that you have done your best for your child, but when things were at their worst, I would feed, clean and clothe ds, put him somewhere safe - eg playpen or cot - and go to bed and cry. If he needed me I would go to him, feed or clean or give him a toy or whatever, then go back to bed to cry some more. And so the day went on. Then, at about 4pm I would get washed and dressed to try and deceive dh that I was OK and we had had a 'normal' day. Hardly good mothering. And utterly different to what our days were like when dd was tiny.
Yes, I too felt very guilty over my apparent lack of mothering of ds. But you know what? He is a healthy, intelligent, resourceful boy. He is fine. And what I realised was that I had to forgive myself for my behaviour. You cannot live the past all the time, you have to take what you can learn from the past, and approach everything with fresh hope and an expectation of the best.
In dd's first year or so, while I didn't have PND I was certainly exhausted and perpetually at the end of my tether, and I was forever losing it with ds and shrieking at him. Again, I thought I would never forgive myself, when, later, I saw that he was scared of me, and didn't really trust himself or me. This time, I realised that it wouldn't be enough just to forgive myself, I had to repare the damage.
I went back to something I had learnt in counselling during PND - concentrating on what I wanted to happen, rather than what I didn't want, and finding the steps to take to get it to happen. I also read a lot of parenting books to try and find better strategies for dealing with the situations that made me lose my rag. Thirdly, I dedicated time to ds alone. I organised ds and dd's nursery days so that one mornign a week he was alone with me, and those mornings were never used for housekeeping stuff.
It worked so well! Our relationship blossomed. Ds learned again to trust me, and to feel comfortable with me. His self-confidence grew, as did his general happiness. And his behaviour with his sister improved as well.
And I finally forgave myself for my bad behaviour with ds when he started Reception 3 wks ago and settled in immediately - happy, confident, proud of himself, making friends easily - all the things I worried that he wouldn't succeed at.
I think you need to give yourselves credit for being good mothers despite having PND, and forgive yourselves for having had it. After all, you wouldn't feel guilty for having had a broken leg and therefore having been unable to dance with your children - you might feel bad about it for a while, but eventually you'd rationalise it and it wouldn't bother you. PND is just like that - only much much bigger.