As a horse person who has been 'over horsed', I can tell you exactly how that happens, and what happens once you are. Here's a little bit of background on me; I am eighteen and have been riding horses since I was ten. I started out taking lessons and gradually became an intern at a show barn. By gradually I mean in a year, not two or three years or even longer.
How does this start? Well, it can start differently for different people and there can be many parts of barn life that start it. For me, it started out as dreading to even see horses. This was a problem as I live in the barn and have daily chores to do in the barn. This dread started because I was told I had to act a lot older than I was, I was given three plus horses to ride, a whole barn to feed/water, and at least six stalls to clean. I was fine with it, for a while, then I started to get burnt out. I was working way too hard, and the drama in a horse barn is extreme.
Which brings us to the next part of being burnt out with horses. I started to hate the barn, the barn had gotten too big with way too much drama for me to handle. Too many people complaining and not pulling their weight when it comes to working at the barn. There was also a lot of 'princesses' at the barn; people who refused to get their own horses ready at shows or refused a lesson unless it was with the trainer. At shows, I had to help get all the horses ready, and I was getting up at four in the morning just to work the client's horses. I was stressed and exhausted, and it seemed like the only one who cared was my mother. Turns out that they did care, they just didn't notice.
I was told I had to ride so many classes on so many horses at shows that I didn't want to show anymore, I didn't want to work horses or ride anymore. I was being a terrible kid, mouthy, irrational, and depressed. My grades weren't falling, but no one wanted to be around me.
Eventually, I started to notice that everyone else was happy and having fun without me; I started to feel left out and like I was alone. That was the worst feeling in the world. Or so I thought.
Turns out that there is a worse feeling; feeling numb to everything is so much worse than feeling alone. People start to look at you strangely, so you learn to hide how you feel behind a mask of happiness. Thankfully, this stage didn't last long for me. I've started college and have found new things to keep my happy, I still don't particularly like riding, but it's bearable. It also helps that while there is drama, I'm not around it so much because I no longer show and the barn has less 'princesses' in it. Please don't burn anyone out when it comes to riding horses, it's a wonderful thing that can quickly become terrible, especially for kids and young adults.