First I have to be responsible and ask, Are you still drinking? If you are you need to stop, you'll hurt the baby and its only going to make you feel worse.
Second do you have anyone to help you raise this child? If not then I urge you to find a support group for single mothers most YMCA's offer them.
Now, find a councelor or someone to talk to about how you are feeling. There is no definitive connection between depression and post partum depression but you seem like a good candidate and getting help before the baby is born is the best way to get through it.
I've dealt with a lot of death, and I've lost people very close to me. I'm not religious but I truly believe that they know how much I love and care for them, and know they would want me to keep going.
You shouldn't blame yourself, depression is an uphill battle and some people aren't strong enough to fight through. You obvously loved and cared for him in no way could it be your fault. Most people can't see the signs.
I've dealt with depression and loss most of my life, feel free to pm me.