Response is at the bottom

Background; I "share" horses from a lady which basically means I pay to ride them, but I also do yard work on top of payment. This extra work started off basic but it seems I've been doing something wrong every day lately as I get a message telling me something else is wrong. Keep in mind these horses arent easy horses and they're attached at the hip so wherever one goes another goes so its double the work all the time. I'm sick of getting these texts every single day telling me something else is wrong and I've decided I just don't want to deal with it anymore as I'm losing sleep over them which is stupid as they were only suppose to be a hobby for 2 days a week which have turned into every evening 7 days a week so I'm always sorting them in the dark and that doesn't make it easier. If she was paying me I'd understand but I'm paying her and I've never let her down. She has also only recently become this arsey with me and I don't know why, I've been doing these horses since August 2020 so it isn't a new arrangement nor am I new to horses, I do know what I'm doing.

Main; her latest text was about how the horse me and another girl ride isn't progressing very fast. She was blaming me for everything as usual so I've drafted a message to send her (names changed)

"I've been thinking a lot about what you said the other day and I just don't think I'm a good fit for Luna if that's how you feel. I love her and riding her, but I don't want to ruin her. I'm going to let Jess ride her all this week and see how they get on and then I'll bow out and let her take them from here. I do love the girls, but they're causing me more stress than I'd like and I've been losing sleep over this, I just wanted some horses to have fun with a few days a week nothing too serious but serious seems to be what you want at the moment. Thank you for letting me share them, and sorry for any inconvenience I've been."

Can anyone tell me if this is an appropriate response, I don't want it to end on bad terms but I do kind of want her to feel a little bad since she's been an arse lately. My anxiety has been horrible over what to say and how to get out of it, especially since I really like the yard and everyone else there so I don't want the owner to slag me off to everyone. TIA