Like most things in life, it's difficult to really understand something unless you have experienced it for yourself. This holds more than true when dealing with serious situations like giving birth, losing a loved one, or having a special needs child. You can hear others talk about it, but you can't really "get it" unless it happens to you.
I don't mean for the title of this post to sound harsh. I love my typical mom friends. It's nothing they have done or not done. It's that they haven't experienced raising a child in the same way I have. I can't relate to you either. We're both moms, but that is about as far as it goes. We each had a baby, but everything from that moment on has been completely different.
I know it's awkward for you to know what to say or which questions to ask about my child. Honestly, I wouldn't know what to say to me either. And when you talk about your own children, you run the risk of making me feel bad about what I'm not experiencing as a typical mom. Which you probably do, but it's not your fault.
Do you invite my child to the birthday party knowing she can't play with other kids or join in on any activities? If you do, I might feel bad being reminded of that. If you don't, I might be mad that you left her out just because she's "different". Either reaction is possible depending on the day. But it's not your fault.
For my child's birthday, do you give her an age-appropriate toy that you know is too advanced for her, or a baby toy that you know is probably more on her level? Either way, I'll be reminded how severely delayed she is and unlike her typical peers. But it's not your fault.
I know that you are damned if you do, and damned if you don't. You are set up to fail. But it's okay. All I really care about is that you are still around after all and that you make an effort to be a part of our lives and include us in yours. So you see, you really cannot win. But just remember that I know it's not your fault.