I think Dixie is spot on. I just want to give you a bit of insight into our journey (my journey)...
Our first year with Lily was really hard. I blamed myself a lot... for everything. And, I got so neurottic about Lily's therapy that I'd get mad at Lily when she'd fight me. So, I worked on finding a balance. Eventually, Lily got her CP diagnosis, and that helped, a lot... finally, I knew the reason Lily wasn't meeting milestones was because she had CP, and not because I had failed her, or was a terrible mother.
But, with that said, I still believe Lily would not have CP, or any of the problems she has today, if it wasn't for the fact that she was born so early. And, the reason she was born so early was because my body failed her (i.e. I failed her).
I AM the reason my daughter is the way she is. And, most days, I am okay with that. It's not so much a negative comment as it is a statement of fact. FACT: At 26 weeks pregnant, I developed HELLP syndrom... there wasn't anything I could do to prevent it, and there was nothing I did to cause it... it just happened, and Lily had to be delivered. Eventually, Lily's prematurity caught up with her, and now she has a whole host of issues that we must contend with. END FACT.
However, I will admit, there are still a handful of days that I blame myself in the self-depricating way you are describing.
I think one thing our children teach us is that we aren't in control. Even if you had eaten better during pregnancy, or done XYZ, that doesn't mean your son would be any different.
One thing my husband always says when I am feeling down is, "You aren't a terrible mother. If you were a terrible mother, you wouldn't care whether or not our LO was behind, and you wouldn't care if you were terrible." He's right... and that goes for you too!
Your son is still young, and your wonds are still fresh - time alone will help you feel better. However, by all means, don't let that stop you from seeking help with your depression. I have often sought comfort and clarity in therapy, as I have battled several bouts of depression. In the meantime, try and identify the good things about your son... as opposed to the negative things. And, by all means, claim those, because, most likely, those attributes did come from you or your H.
For example, while I can tell you my daughter is delayed due to her CP, I can also tell you her beautiful smile and kind heart come from me, while her joyous nature and expressive eyebrows are her fathers.
I would also suggest you take some time for yourself. Yes, it is hard, but there is a benefit to reclaiming who you are as a person, and not just a mom. Because, if all you are is a mom, and you believe you are a terrible mom, then you begin to believe you are just terrible. However, if you can say, "I am an awesome XYZ," then you begin to gain some perspective on your child's problems.
I am sorry, this turned into a long ramble, I did not mean that to happen. I just want to add one thing... I picked this book up at a local used book store sometime last year. And, it was AMAZING. It helped me get through many of the feelings you are feeling today. I suggest you check it out.