OP, I had a similar experience to you. Always planned to have two, but didn't due to secondary infertility. I found it really hard to come to terms with initially, and I felt so guilty.
DD is 15 now, and she is honestly the happiest, most well-adjusted kid I know. Incredibly confident, fantastic social skills and tons of lovely friends. We have a really close relationship - much closer than most of her friends are to their parents. She has never wanted a sibling and considers herself lucky not to have had to deal with sibling rivalry, constant comparisons, siblings who don't get on etc. She has also benefited from more time, attention and financial resource being lavished on her. From her perspective, there are genuinely no downsides, and if I could go back now and change how things panned out, I wouldn't...not because I didn't want another child for myself, but because I actually believe now that dd has been much better off as an only child.
I do still have some concerns about what happens when we are older, and whether dd will find it a burden to be the only one to support us. However, I bear the entire burden of caring for my own elderly parents despite having a sibling, so there are never any guarantees. We are also planning ahead to ensure that we minimise any burden as much as possible.
Grieve for the child that you couldn't have, by all means, but grieve the loss for yourself and not for your existing child. I have learned from experience that being an only child can actually be an incredibly positive thing, and from the perspective that I have now, I am actually grateful that we were able to give that gift to our dd.