From the sounds of things, it's somewhat a game to him, and he may not realize how much you mean to him. I don't know his age, but I know you are not very old and at that age, I couldn't say I knew even half of what I know now about my own emotions, let along guessing at others'.
If he is this intolerant at this point, and wanting you to give things up, it's not going to get much better with time unless he is willing to admit he's wrong about it and make some changes to accomodate you.
A long-term, successful relationship is not necessarily a 50/50 deal. It is a series of adaptations, of sacrifice and demand. There has to be a willingness to at least listen and consider, and an agreement to "try."
A good communication session is in order.
The horses should be off-topic as they really have NOTHING to do with your relationship between he and you. If he gets onto that, just ask him to address your relationship as if they didn't exist, and in turn, don't bring up HIS passions.
My husband and I have pretty different interests though he goes far out of his way to make sure I enjoy mine. He loves the social aspect of the shows, he even enjoys the competitions where we are involved and seeing the foals born from my choice of matings, etc. He doesn't really like that they keep us tied down to our home as far as being able to leave for stretches of time, but he deals with it just fine.
He really doesn't "harp" about them, either, much, and I don't ride him about his enjoyments even though some of them aggravate me (like collecting junk cars *LOL*).
Truthfully, if I start to get upset over something he enjoys, I think it over and it's usually more about my mindset towards HIM at the time than anything else, and vice versa. To bring these blameless "things" into the mix is just looking for excuses.
You are young, and I know it seems hard, but try to see beyond the immediate future. There are many options, here, and if it does not work out between you, you will be happy in another incarnation of yourself. It does not take another person to complete you nor to make you happy. You can be and need to be happy just being you for the sake of you. Once you know this, I think you'll find you will have your pick of well-adjusted admirers.
Not telling you what to do, just trying to be supportive. At your age, two years is a very long term relationship, and I can see that it is not easy to give up on something that deeply ingrained.
Best wishes,
Liz M.