No course not I'm totally with you on this.
I'd go ballistic if anyone did anything with Kelly tbh without our permission...but this might be over protective 2nd mum syndrome
You don't need to feel silly for feeling like that - I'm sorry but you're giving your time, knowledge and even your horse to people who need help or just the odd jumps (I'm totally with her on that too btw as I'm currently jumping my YO's horse with Kelly being out of work, I miss it too much) but if ever YO turned round to me and said sorry Kerry's not available today, it'd be a case of okay fine no worries maybe next week. Its YOUR horse, your decisions and you don't want to be risking injuries because this other girl wants to ride. I take it her share horse doesn't jump?
Mayb you should send her an email and say I didn't mean to cause problems, just that I don't want to risk injuries with my horse and he's NOT an easy ride over XC - and say you're happy to reschedule in the school and just reiterate everything you said - maybe leave it a few days until she's calmed down and then send it?
Other than that and not feeling bad yourself, the ball's in her court I think...you've been polite enough and handled it well. As for the woman who needs help with her horse - like you said you aren't going to be there everytime!!! She knew that, you knew that, don't feel bad - if she wanted to ride she'd have had to learn to be strict there and then (I know it sounds harsh but she can't expect someone to be there and help her everytime)
Just to add...I've been riding for 12 years now and YO said on my first lesson with Kerry we'd only do a few tiny jumps as I needed to learn how to control her and ride her to my advantage first off! She surely can't expect to get on your horse and go??
I have helped many people in the passed only for it to be thrown back at me (even verbally abused and threatened once!). I am now more careful who I help and am quite selective. Keep my head down and am pleasant, just don't go out of my way exceot for close friends.
It's sad, but after what happened the last time, I'm just not doing it again.