Horses are a bit of a pain in the ass when you get started since the controls aren’t explained. Here’s a couple tips I’ve learned myself that could help others.
General Tips & Tricks
Horses don’t really have a “Throttle”, so you have to hold W to continue moving at a trot. Press shift to kick things into a Gallop, but be aware it isn’t as easy to turn this way.
Speaking of turning, this is where it’s most frustrating. Turning is based on where your mouse is looking, but I think doing a combination of that and using A/D can help turn a bit more. In order to make a sharp turn, tap/hold S or reverse while turning your camera in the direction you want to go. Your horse will whinny a bit and should turn much more quickly in the desired direction.
If you find yourself stuck facing a wall or stuck on some geometry, hold S and use either A or D to strafe away from the obstacle.
Hope this helps!