We are one and done. Like you, i'd always imagined two but we just never found ourselves in a place where we wanted to take the plunge. I found the baby stage hard, and we're both a little older than we expected to be having a child so, to be frank, we're both knackered. I've also really struggled with the additional organisation etc. Mentally, it's been hard.
I do really love our life, just the three of us. But I do feel a little bad, because, now I'm an adult, I really love having a brother and I'm sad that DD will never have that sort of relationship. However, her cousin is also an only (and none at all on the other side) so my brother and I are being really proactive in ensuring they have the chance to develop a close relationship with each other. Also, there are loads of only kids around these days - Dd's experience will not be a rare one.
I also slightly struggle with the idea that I "gave up": having kids is too hard for me. But that also means that we've made the right decision for our DD - we can be the best parents for her that we can be with the resources we have.
There are, of course, plenty of positives! We can focus on her but also, now she's a little older, have time to ourselves. Our attention isn't split, more money etc are also useful!
I think, in the end, neither choise is perfect. You just have to lean into the positives whatever you end up deciding to do. Xx