Interesting reading these!
I feel sad on/around my birthday, too (also my children’s birthdays, and Christmas although with Christmas increasingly the business/tiredness/relief cycle overshadows any actual emotion!).
For me, it’s definitely not to do with it being a reminder that my life isn’t what I expected, or would like, it to be - on a normal day to day basis I’m actually really content and feel truly fortunate for what I have. Some of it is perhaps singleness - maybe this is a day when having a partner who (fairly obviously/uncontroversially) ‘should’ make it feel special, makes a big difference; but again, in normal life I’m perfectly happy being single, and actually when I was in relationships I still had morose birthdays!
I do strongly recognise not feeling comfortable being centre of attention and perhaps this is part of it. My childhood memories are very, very limited (a psychologist I briefly spent some time under regarded this as significant, but I’m less sure) and I don’t really remember what my birthdays were like, I can remember having parties and my mum baking a cake but little else, good or bad.
I’ve done a lot of work on being able to communicate directly what I want, but I’m still not reliably good at actually identifying what I feel or want - this may also be part of it I think.