Hi. I'm 39.
I feel the same way. Whenever I plan for activities, I plan to do them alone. Even my equipment reflects that. I bought a kayak instead of a boat. I took the foot pegs off my motorcycle, which I ride alone. I'm thinking of building a small cabin to live in. I go on poker trips alone. I moved to Costa Rica alone. I did remote work with no pairing.
I don't feel too lonely, but I do occasionally. It comes in waves. Don't get me wrong, though, I get out to a lot of events. I spend the majority of my days totally alone with my dog, but I spend many days around lots of people (I own a homestead, and a bar--two wildly different atmospheres).
That keeps me...whatever. Sane, okay, socially fulfilled.
People like me, I could be more social. But this is my level, I don't know. If I add to it, my life feels worse.
I do want to say that I really enjoy my life. It's not normal in many ways, but I'm actually quite happy. But it wasn't always. I've been very lonely in the past, living in this same place. I think a big realization was realizing that this is normal. This is the human experience. Loneliness is part of our programming. We're intended to live together. Those of us who choose to go alone are going to have to battle emotions and situations like this. And it's okay. I usually acknowledge it and then go do whatever the fuck I want to do, because I don't have anyone to answer to. And because I chose it, and continue to choose it.
My advice is to embrace it, and be your own best friend. Or abandon it, and go find your people.