I understand what you're feeling. With my Dalmatian, I kept going back over (and over and over) things in my mind, second-guessing the decisions I made during her final illness and beating myself up. I did that for a while, then had to sort of kick myself out of it by reminding myself that I did the very best I could with the information I had. "Could've, should've, would've" wasn't helping me, it was hurting me.
I tried to do what was best for her, not for me. You did the same thing. Remember that your baby knew you loved him and remember the good times. It's never easy, in fact it stinks that we have to make "the" decision.
You are not alone. Talking it out with good friends can really help. Composing a rememberance can help--here on Greytalk and/or on other tribute sites (I did one on In Memory of Pets -- http://www.in-memory-of-pets.com/personalt...te.php?ID=42437 ). Also, sometimes you just have to let yourself cry and shout because it's not fair.
My thoughts are with you.
Beverly