Yes, I am an only child.
I’ve never felt normal either. When I was a kid I always felt more comfortable around adults and less comfortable around children my age, which is apparently a common characteristic. Maybe it’s because as an only child, you’re less constantly ‘exposed’ to other children of a similar age since you don’t live with any, compared to the overwhelming majority of kids who do? It might explain why I was very, very shy at school and felt less ‘with it’ than everyone else (and why early secondary school was so intensely horrible).
Socially it’s shit, and I’d say it has messed me up. On the other hand it’s a luxury because the resources and attention of your parents isn’t divided and you never have to share anything. Definitely worth it overall.