Hello OP
Could have written your post almost word for word, especially the part about feeling you might not have the energy/enthusiasm to do it again and be as good a mum!!
We have various reasons for (aaaaallllllllmost certainly) deciding to stop at one: health/diability (the major reason), finances, age, and the fact that we are actually very very happy as we are.
I am 39 and think the window to make this decision is getting smaller and smaller - I don't want to end up making the 'decision' by default but it feels hard to make the final decision iyswim.
in essence almost all of me - head and heart - says it is way way better for us, for all our reasons, to stop with just the one.
But like you I can't shake the lingering worry that we are failing DD by not 'providing' a sibling/playmate for her.
Logically I know this isn't true as I KNOW siblings can loathe each other (have read anough of MN and seen enough of this in RL to know for sure this is the case very often!)
I think what I feel DD is 'missing out on' is some sort of theoretical best-of-friends-for-life relationship with a sister (or brother) whom she adores.
Logically, this might very well not happen!! DD is a jealous type to say the least and honestly right now if I were to walk through the door with a sibling for her, her little life would actually be destroyed (even if only short-to-medium term!!) as she can't bear to have my attention off her for one single minute. Yes, yes, of course, she owuld get used to it and it might well be good for her, but that doesn't change the fact that I actually think some children are way more cut out to 'share' their parents than others. Personality type is vastly important here!!
But anyway apart from this occasional desire to give her a sibling I pretty much have no desire whatsoever to do it all again.
So not much help OP, sorry, as I'm really in the same boat as you!
If it helps at all I have raised this quite openly now with most people who ask if/when we are having another and I've been amazed by how many people have told me they wanted to stop at one, too (but didn't, for various reasons, quite often simply expectation of others). Not to say for a moment that any of them regretted that second child - they didn't - BUT it has been interesting to hear other women saying they really weren't keen/that bothered about a second child, they often just felt they 'had' to.
But that argument could go the other way because obviosuly all of them adored their 2nd child so maybe I am just hearing what I wanted to hear from them, which is that only wanting one is very common and normal!!
Good luck OP with whatever you decide. If it helps at all I gather that 1 child families are getting more and more common so if you stick at 1 your DD won't (depending perhaps on where you live) be the only one in her class etc...