I always feel sad on DS3 (ASD) birthday. I can always remember what DS1 and DS2 were doing at the same age, the party they planned and had, the friends, the excitement of the presents, the anticipation for weeks.
DS3 will be 6 this month and he does not know or care it is his birthday. No nagging me for what he wants. We will have a tea party but make it seem as un-party like as possible as he hates parties. He may not open or show any interest in his presents. It will be just another day.
Milestones are tough for children who miss them. I used to feel guilty that I did ever reach the point of accepting DS's SN. But there is enough to beat yourself up about. I have sort of reached the point where I accept that I don't accept it, but as long as he never knows that, thats ok. I don't think its odd to want your child's life to have been easier.
I was brought up on neuro-typical birthdays and bought into all the hype. I know DS doesn't miss all the razzmatazz but I am not autistic and I do miss it, for me, not for him. And I think thats ok too. We had certain expectations and have had to sacrifice things which are important to us, even if they are not important to our children.
I have a video of DS at 18 months old, before he regressed and when he still made eye contact and played with his grandparents; and even now I can't bring myself to watch it.
Be kind to yourself, it is ok to feel sad about it.