I nearly always ride alone. I'm a loner so I don't really like company, unless I'm training, in which case I'm better at sticking to it if I have company (and someone to one-up!).
On trails, 99.9% of the time I go alone. That way, I turn back when I want to, I go my own speed, and I don't have to worry about what to do if my friend gets hurt (the person I normally ride with is 14 and I'm now legally an adult, so I'm responsible for her, and her actions). If she's at my place, my mother is the one legally responsible for her, because my mother is the oldest and most mature person here. And I'm not allowed to bring minors onto the property if Mum's not here, because of the legalities if someone gets hurt.
My horse is a funny one. He likes company, and he likes solitude. In company, he likes to show off, and he can get strong. Alone he is much easier to manage. But he is happy either way and if I just let him go he would match the other horse's speed anyway... once he got level with them.
I'm not a confident rider, and have suffered from all sorts of issues because of it. When I'm riding on trails, I usually stick to walk with a bit of trot and if I'm feeling really brave some canter where the footing is best. There is one trail I can gallop on without worrying about traffic or whether he will pull up in time to cross a road, but I don't ride it very often because I had a bad fall on it a while back where I cracked my helmet in two and fractured my humerus... so my confidence issues are at their worst there, and I refuse to even trot in the direction of home because the fall resulted from horsey getting a bit jiggy, so I let him trot a bit, and he broke into a canter, then a gallop, and in the end I came off. I will gallop away from home, but walk only back towards. Even though said trail is an hour's ride from my place anyway!