There are far too many people out there who socialise out of a sense of duty. I am NOT one of them. What exactly is the problem with not being arsed to join the tinkly laughs and faux jollity of a social engagement, when the first thing you want to do is get out of there pronto!
Be happy in your skin, like who you are and try and embrace yourself and YOUR wants and needs, not those of others. Say no thanks once, and it is so easy the next time.
As for being lonely in old age, well that's not something I worry about right now, I could be dead next year and would have wasted most of my time left going to things I don't want to! When I am old I'll worry about that and I can afford to pay for a companion if one is necessary. EFWB elderly friend with benefits, LOL.
I think being able to say no, and turn down or dodge invites is just so empowering, and there is nothing like the relief of not having to perform at various "things" that are largely forgotten about by morning anyway.
Yes keep in touch with people by whatever means works for you, meet those whose company you enjoy but dodge anything that gives you any stress, worry, tiredness, anxiety, or sheer boredom and you don't need to explain anything.