I think I went through something a bit similar when I hit that age. I don't know your situation, but I didn't really have any horsey friends or people that I rode with, and I started to focus more on other things for a while, but like you I did still have fun in lessons.
Personally I pushed through it and kept riding, because like you I loved my horses and I knew that they needed to be kept in work and excercised regularly. I found that that after a little while my enthusiasm was back and stronger than ever. Riding and spending time with my horses is now as important to me as anyting.
As long as you do still enjoy lessons I would keep riding and see if it is just a phase you are going through. That said, please do bear in mind that if you really have just lost interest in riding all together, the fact that you love your horse is not enough. You have to put his needs first and ensure that he is excercised and stimulated regularly. If you are not interested in riding him, he should be with someone that is.
Perhaps you could look into ways to keep your interest going. You could consider joining a pony club or riding group or finding someone new to ride with? Or perhaps you could look into trying a new discipline, like jumping for example. I don't know what kind of riding you do but if you generally do the same thing such as flatwork or hacking, trying something new might just grab your attention again.
I hope you get your interest back, because you are young enough to still be learning and progressing, and this is a very rewarding process if you do hang in there, however if it is just not going to happen, don't force it.
~The most important lesson my horse ever taught me is how much I have left to learn~