I'm sorry to say I'm only preg with my first and I don't know a lot about PPD, but I definitely agree with blueaquaqueen. I think a lot of parents worry about being bad parents, but it's the ones who don't give a crap at all that actually ARE the bad parents. From what I've been hearing from the reminiscing older couples, a lot of things especially with the first child are trial and error, and a lot of mistakes are made. I know that's not what you're asking... But my sister had her 2 girls close together, and she was sooo sick with hyperemesis (really bad morning sickness) that because she was on an IV she wasn't even supposed to lift her firstborn. But she still did, and they still spent lots of time together before the baby was born and even after, even tho the 2nd was extremely fussy in the beginning. Now the oldest is almost 3 and her sister is almost 1 and a half, and they play together wonderfully. She didn't miss anything in the older one's life except for the times she couldn't get up and chase her quick enough, which I think is understandable and not in any way bad parenting. It's normal to worry and have fears, but people have close-together kids all the time without losing anything but sleep when they won't take naps at the same time.
I hope everything goes well for you and your son and the new little one, you don't have to be superwoman to be a great mom.