Oh OP, you sound just like me 2-3yrs ago. I felt sure I was going to drop off a huge cliff in my own head, somewhere no one would ever find me.
No matter how much people said, "you're not alone", I was in there.
It took me 2+ years to get help from my GP. I knew it wouldn't help but I was down to 2hrs sleep a night, I was hallucinating with tiredness, and I hated everyone, mostly myself for ruining my life and my baby's.
I wish I'd got help sooner, OP. Even if you know (as I did) that it won't help, please go to your GP and lay it all out, as you've done so bravely here.
Meanwhile, do whatever you can to keep yourself and your baby safe. Put the baby on the floor, walk away. I used to whip the walls as hard as I could with a towel, screaming with anger. Smash glasses into the bin. Throw rocks at your garden wall. Tear up old clothes with your bare hands. Redirect that tiredness and heartache and anger to something else until someone can help you.
So much love to you. It does get better, I swear.