WME - that name is sooo wrong! - I've just read this thread, and want to say how sad I feel for you, and how much I hope things improve very soon. and to send you a very big huuuuuug.
I went through this with my first ds, had a very, very similar experience to you, had a very, very similar response, and totally understand your feelings: they are due to depression, and that's due to all the stress you're under - the reason you feel like this is NOT because you are a bad mother, or a bad person: your pnd has arisen for many reasons, and can be resolved. I found there was no magic wand to relieve my depression, but you've taken a HUGE step coming on here, talking to people here, then going to your doctor: you're keeping going. I know things will improve, they really will, it just takes a bit of time and support from others.
Shame your ds's idiot, selfish dad isn't one of those helping 
Taking one day at a time helped me enormously.
I was very struck by you saying you don't feel you can give your ds enough. Really, small babies don't need a lot: if you're still bf, amazing, and big congrats from me; your baby is having that physical closeness with you, as well as nourishment. He'll be fine - it's you who needs to put yourself first so you carry on for him. I used to feel consumed with guilt that I was terrible mother when my ds cried, and he seemed, like yours, to just always be crying. By 7, 8 months old I was in despair, wanted to do anything that would stop the crying, wanted to escape (permanently), felt like the worst mother in the world, got into a complete horrible head spin about everything, it was simply awful. I had never been so tired, ds didn't sleep more than 2 hours at a stretch day or night, and I felt like I was plummeting down like a stone.
My first solution was surprisingly simple: my lovely kind HV suggested co-sleeping. I read a book called Three in a Bed which gave me the confidence to do this, and within about 7-10 days ds had calmed down at night, cried less, I snoozed more - ds still didn't sleep through, but he was happier, and I gradually relaxed, and stopped being so utterly exhausted. I can honestly say this was a total life saver, especially saving my life, I can't recommend it enough when you've got a baby like this.
I also had counselling via my gp which helped me understand I wasn't the worst mother ever; that was another life saver for me. ( I can't take ad's for medical reasons).
Lots of small steps add up to a path through.
BTW, my ds is 20 now, sleeps through the night happily, is a fantastic young man, and was a fantastic kid once he got past those very early years. Well, he was always fantastic, but it's hard to see that when you're depressed and he keeps crying!
I hope you're getting through your weekend ok; sending you very warm wishes x x x