SassySusan, I am so sorry you are going through this. Christ. I can't even begin to imagine how hard this must be for you .
First up - YANBU. You sound full of rage and, at the risk of sounding patronising (please forgive me if I do) this is a natural part of grief. I don't blame you for wanting to slap every one of these mother's who seem to be going on with their normal, every day life when you have experienced the loss of your precious child.
The terrible truth is, there isn't anything they can say to you to make any of this any better. And I do believe that most people genuinely don't know what to say in these circumstances. It is a huge taboo - the death of a child. What is there to say? You are very likely right - you probably do remind them every time they see you that this could have been their child, but isn't, and it throws up all sorts of uncomfortable and awful feelings and thoughts.
God, it is awful. It must be so painful for you. Do you have a good, close friend who you can talk to, or just be with, for support? Someone to face the world side by side with, and fend off any clumsy or misjudged comments on your behalf? You have enough to deal with without having to face people every day that make it all feel even worse.
I don't know what else to say except that YADNBU. You are in pain. And I am happy to listen to you, whatever you want to say.