Is this your first child?
Having a baby is a massive massive shock to your self identity. You might have had a vague idea who you were before...you might have self-identified as being a competent worker, a good friend, a strong woman, a loving wife etc.
Then suddenly you get dumped with 'mother'. You have no experience, you don't know if you are any good at it. It might fit you as a concept, but it might not. You might find that struggling with 'mother' means you start to struggle with wife/friend/competent worker....
Suddenly it looks like your shit at everything. No place you can turn without feeling you are failing.
Then you maybe start to withdraw...because you aren't doing your kids/husband/work/friends any good so why inflict yourself on them. You are tired ALL THE FUCKING TIME and you don't have the energy to get out the house. And that's when things really start to feel bad....
I am of course telling you how I felt with PND....not how you are feeling!
So to answer your OP, two things turned it around for me.
Firstly my therapist explained that the urge to withdraw...to think...to understand and constantly replay the ways in which I was failing, was hurting me. It was making things far worse over time.
I was so desperate to feel better I was prepared to go against all my instinct and attempt to re-engage. To go talk with friends even though I was pretty sure they all hated me. To spend time with DD even though I thought she would be better off with me dead. To just do the things of the moment that I felt the urge to do....like 'I'd like a cup of tea'...don;t think about it, just do it. I'd like to go outside and lie in the snow...just do it...don't think about it. I re-learned how to say yes to myself. Over a few weeks this change in approach caused a change of behvaiour which led to a change in thinking and finally to massively improved mood.
The second thing was the 'Mother' guilt. I absolutely knew I had been a terrible mother....I had let down my child...I had not been there for her when I should have been...I had not loved her like I should have loved her...I had fucked up everything for her. (My DD was 3 by the time I got the help I needed).
Between me and my therapist, we laid out some actual facts of what I had done. He made me realise that going through the motions...feeding my baby, changing nappies, holding her while she cried...everything I had actually done, was not in fact 'letting her down' but doing the absolute best that could possibly be done by me in the face of the mental illness I was suffering. He turned every failure I perceived into a battle that I had had the bravery to fight when all I wanted to do was run away from it all.
So in conclusion after 3 therapists who told me to think positive thoughts and spend some me time...I got a therapist who changed the way I was reacting to my depression and who unpicked my personal history and recast it as something I could live with as opposed to something that was killing me.