Honestly, I understand. I was so upset about secondary infertility (and to a small extent, I still am). One child is not what I would have chosen. I'd have had two (not greedy either!).
BUT now my child is eleven, and is happy, and has plenty of friends, I realise it's OK. Now there is no way I'd want a second! I've never even got a dog, haha. I've just ensured he has as many friends as he likes (he's very sociable). Plus, we have such a great relationship - not ever something I had with my mum, harassed as she was with several kids. You really can have such a lovely relationship with your child.
I've also noticed that every child has something tricky, or something 'outside' the supposed norm - - whether it's a poorly sibling, or a sibling who bullies them, or a horrible parent, or bullying at school, or a million other things. To my mind, the main thing is that they are happy. And I can say that my child is incredibly happy. He likes his life.
Also, it's all he's ever known, and so he 'owns' it. Having no siblings wasn't my childhood experience, and I'm guessing it wasn't yours. In fact, as a kid, I thought only kids were a bit 'odd'. It's simply not true - they are just kids and like all kids, you get the odd ones, the loud ones, the kind ones, the quiet ones. However, research has shown only children tend to be happier! Actually, dare I say that of all the primary school kids I know, there are two that are a bit mean (little turds actually), and both are the older of two brothers... but that's just my anecdote. So anyway, it's your child's life, it's your life, I hope you enjoy it and don't spend too much time (as I did) lamenting things that can't be. The alternative-what you have-can actually be really, really nice!