Thanks for your comments yesterday everyone; I'm glad I'm not the only one is ok with setting boundaries with trainers.
It rained all night and I cleaned this morning, so by the time I was done, I didn't really feel like trekking over to the Doom Bubble to ride. Instead, I turned some of the girls out in the arena. Cassie, my old TB mare, and Ellie, her Hanoverian friend, are probably going to be ridden by a couple of my friends tomorrow, so I figured it was best just to let them get the willies out. They had a ball galloping around in the mud. When they were done, I groomed them and put them away.
I turned Izzy out by herself because I don't really trust her loose with someone else's horse. I'm always afraid she'll hurt them and I'll be responsible. So instead, Izzy and I did a couple of leading exercises. She did a great job just responding to my body positioning; I didn't even have to apply any pressure. Then she ran around like a maniac and I let her go back to breakfast. I figure the day off will do her good. It's too easy to get excited about the progress we're making and forget to just let her be a horse. She's still not back out in the pasture because of all the rain and mud, so she does need to get out every day and do something or she gets really bored and nutty.
This summer, I'm hoping to be able to trail ride her at least once a week to give her a mental break. Plus, when in heavy work and out in the pasture, I usually only get her out 6 days a week, so she can relax.