I feel the same way and I am 40+ years old.
It used to be that I didn't care a whole lot if I rode alone. And from a comfort-level with my horse perspective, I don't. But I've ridden alone so much the past couple of years that I really MISS riding with someone. Someone to share my love of horses with, someone to discuss tack and feeding with, just someone to share nature with.
My best riding buddy is almost 80 years old. She is great. But she's had a few health issues and the weather has been yucky so she hasn't been riding lately (basically all winter) and I just feel so alone.
I used to have friends. It used to come easy.....they would seem to be friendly towards me and I didn't have to do anything special. But a couple of them have seemed to go their own way and we don't talk much anymore. One doesn't even ride anymore.
So anyway, I guess I am no help. I'm just venting. But this subject has been on my mind a lot lately. And then this thread came up. So yeah, I don't have any suggestions but I understand your feelings.
One thing that happened is that I had a young gelding that I raised up from a colt. I had help and he had training, but we never were really quite right for each other. Rides often ended up with me frustrated and crying. I finally re-homed him but sometimes wonder if that's where I killed off all my friendships. Nobody wants to ride with someone that's not fun to ride with.
The horse I have now is great though. I thought once I got her people would ride with me again because "wow, no more horse issues!" But that sadly hasn't been the case. I have an awesome horse and I'm still riding alone.