Yeah you worry too much. If its any consolation most people do, regardless of what it is about.
But think, your life must be pretty good if all you have to worry about is your horse.
He's a horse, they are pretty solid things, they have been living in the wild for hundreds of years. People go on about how sensitive they are too feed and I guess they are to an extent, but not really to the extent that people make them out to be. If you be sensible then things are generally okay.
Just chill out, be smart about things when they arise, but don't worry now.
I'm sort of a "serial stresser" where I always find something to stress about, but I have gotten a bit better. The best thing for me is putting things in perspective, and it jolts me out of it and I see how ridiculous my state of mind is. Why stress about having to move out early and have nowhere to live if I can afford accommodation? Many people out there can't and they are perfectly fine. Why stress about schoolwork? Just do it, worse case scenario I fail and even that is not that bad.
Things aren't really as bad as we think.