Our DD is an only child, not really by choice as both DH & I are 1 of 2, so both thought 2 would be the natural thing to do, I fell easily with DD, but as an older Mum (much older than you) knew we were very lucky -
it just didn't happen a 2nd time around & niether of us felt IVF etc was a route we wanted to go down - it happened as Nature intended - or not at all - & now, even if it were possible I realise I wouldn't actually want to go back & cope with the whole baby thing again, health problems would make that very difficult anyway.
I do sometimes feel sad for DD, as she has no close Cousins either, but she is bubbly & very gregarious & makes friends very easily & has a core group of close friends since she was very young, she loves her own company though & is happy to entertain herself for hours on end, so as much as part of me wishes it were different, I think she will be okay.
One of our biggest worries was, that out of our circle, we know only 2 only Adult Children - both are to put it mildly high maintenance & very "Princessy" expecting they are the centre of everyones world& prone to throwing fits if not
- We would HATE DD to grow up to be this way, so have made a big effort o keep her feet firmly on the ground & make sure she mixed well with other kids from tiny.
Only recently I bumped into an old friend I hadn't seen for a lot of years, & we were comparing notes on our Kids as they played together - I hadn't ever realised she too was an only Child, & though very bubbly & gregarious just like our DD, I have never ever seen her as "princessy" quite the opposite in fact -
she told me, that just like our DD, she did at times moan about wishing she had a Sister or Brother, but then saw how fed up her friends often got over their own & how their families didn't have the holidays, or treats that her family could afford with only 1 child & she realises just how lucky she was & still feels that way, as not having siblings taught her to how mix with other kids & as an adult she has never felt lonely as a result, but instead loves her own company too.
As she put it, it is what it is, I'm an only Child too, but have never regretted that at all, as I'm sure your DD won't
Also, I'm not an only child, & though I love my Brother, he rarely speaks to me, causes a lot of trouble as he is quite a messed up & bitter character, who only after I finished work did I realise has some twisted form of sibling rivalry towards me, he wasn't as successful as I was & now resents my having DD too & has taken little interest in her, he was spoilt rotten as a kid & is what our Mother made him, which sadly is a man who thinks he is owed, still takes from our parents to pay his bills as even though he earns well, he just can't manage his money & I know that when they are gone, he will expect that from
So from that point of veiw -I sometimes wonder if being an only child is really such a bad thing at all
I suspect its not, providing to help your Child to grow up to see the good in it & help them mix well :)