None of this has to be framed as right vs wrong. Just “Over time, the time/effort required for horse care that I’ve been providing has increased, and I’m afraid the current situation isn’t a good fit for me.”
Then: “What would make it a good fit is if we can do 1, 2, and 3. Will that work for you?”
You don’t owe any reasons, apologies, or excuses - just “I realize you are disappointed, but I’m hoping we can craft a revised agreement that will work for both of us.” If she gets testy, give her space to process. “Thanks for hearing me out and I hope you’ll give my suggested solution some thought. I’ll circle back in a few days to hear what you think.”
Chances are, she’s not purposely piling more work on you-- she may be oblivious to how much the workload has changed, and in any event, it’s fair for her to assume that you didn’t object since you’ve accepted the changes. But since what you are proposing will increase her costs, it’s always possible she’ll react badly. So I’d have a backup boarding situation already scoped out before having this conversation.
(Likewise, don’t be offended if what you propose doesn’t work for her. This no-fault attitude needs to go both ways. It’s just not a good fit.)