I am an only child OP and I don't feel shortchanged. I guess I've known no different. I appreciate that it can be wonderful to have a sibling, but for very person I know with a healthy sibling relationship I know another person who has a tricky sibling relationship. I am lucky to have a handful of wonderful friends who have my back and they mean the world to me.
I also have an only DS who is almost 18. His father and I separated when he was 5. He's not an only by choice but unfortunately I had a pregnancy related brain haemorrhage.
When he was young I went through periods of awful guilt about his only child status. I spent a great deal of time worrying about it and fearing what the future might bring.
My worries have proven unfounded. In general he has been a happy, optimistic, enthusiastic young person.
Of course we've had ups and downs but that is how parenting goes. We've generally been close, he is my sunshine and my pride and awe at the person he has grown into has increased year on year.
Inevitably as he has grown up he has become more independent so we are close but in a different way these days.
When he was younger I was mindful about facilitating friendships where possible. Although, I tried not to push this too hard as I didn't want my worries about him being an only child to become a self fulfilling prophecy for him. He may, or may not, see it as a negative thing, (and I'd never dismiss his concerns), but I also wanted to model to him that we are a tiny but nevertheless perfectly imperfect whole family with all we need for a good life.
These days he has a wide friendship circle which is important to him and which of course at nearly 18 is not my business to engineer! However, most of his mates are lovely young people and for the last few summers I've driven 4 of them to Cornwall for 2 weeks in the summer where they have had a ball whilst I relax and read interspersed with joining them for the odd walk, surf and meal. Last year another 5 joined us via train and I barely had to lift a finger to cook all week. That week I had a tearful moment remembering the fear and grief I felt for his future when he was a toddler. I regretted the time I wasted worrying about the future rather than savouring the moment. I realised that although plenty of things have been very hard, none of them were related to him being an only child, and none of them were things I saw coming.
Sorry for the long rambling post OP, I absolutely don't want to minimise your feelings. But with the benefit of hindsight my advice would be to try not to let your worries about the future overshadow everyday life. All we really ever have is "this" moment and worrying about what "may", or "may not", be can rob us of living in the moment.