OP, I have an only child. I understand the guilt that you're feeling. Society constantly tells us we need at least 2 DC, 'family is everything' etc. The reality is that there's are millions of only children out there who are doing just fine. There's also millions of people with siblings out there who aren't!
Your DS is 23, you've done the hardest parts! He's also probably at an age where he hasn't created his own family unit yet but he will. Whether that's by getting married and having children of his own or by solidifying his friendship group or finding hobbies that he loves. He will always have connections in this world.
You can put things in place now to make it easier on him as you get older (a will, sorting through your belongings, thinking about your retirement options).
I have a brother and honestly, he's been a thorn I my side my entire life, he's never been of any support to me. My mum has a sister, they've never been close, all the care of my nana fell to my mum, my auntie made things incredibly difficult in the later years of her life. You can't guarantee that siblings will be close or supportive to one another, they're often a source of great tension and conflict.
As a PP said, do some self care today. Your anxiety has got its claws into this and you need to unhook it.