OP. I understand how you are feeling, I went through the guilt for a while at DD being an only child. I was 36 when I had her and suffered agonising SPD and was advised by my consultant not to have another one.
I said to my mum, but she will grow up now with old parents, old grandparents, no sibling, no family....... and my mum said, but she will meet somebody and have her own family..... she has lots of extended family, cousins etc.....
I know of more than one family who have an ageing parent who needs care and it doesn't seem to matter how many siblings there are, it always seems to fall on one to do the care as the others are too busy/working/have kids/whatever... so being 1/2 or 1/3 etc does not always mean that the care burden is shared.
I have a friend who cannot conceive her second.... I said be thankful that you have one, I know of two women who dearly wanted children and for medical reasons were unable to conceive. But I also said to her, that I understand that her problem is very valid to her and people saying to her "think yourself lucky you have one" is not going to just snap her out of it.
I have a DB. We get on ok and he has been very supportive since my marriage broke up and would do anything for me, but we are not particularly close, we don't hang out together.
I would have loved a sister, but never got one. DD would have loved a sister. But she has plenty of friends and if we go for days out, I usually take a friend with us, so that she has somebody to play with.
As it turns out , now my DD aged 7, has now got a sister, thanks to XH and his much younger girlfriend. But there will be a 7 year age gap and she will only see her every three weeks. So she has got what she always wanted, but still won't have that sister to play with all the time.
As a single child, DD is sociable, makes friends quite easily, has a great imagination and is happy to play on her own.
As for people saying "what a shame" - just tell them to fuck off. or more politely you could say, "well it wasn't by choice". and just leave it at that. When people asked me in my early 30's why I wasn't pregnant, I used to tell them that I didn't think I could have kids. They soon shut up then.
I agree that some counselling would be a good idea as well, to talk it through and reconcile yourself to it.