You already know, from your own experience with horses, that there are many teen-age girls who would do your job for free, just to be allowed to smell the horses and maybe get a chance to ride.
So you are looking at competition for your job that costs the stable $0 per hour.
The best outcome for you is that you get to be so well-regarded at caring for the horses that everyone wants to trust you for other sorts of things.
Or from the opposite point of view, I have in my head a list of about 15 screw-up people that I do not want to have on my property ever again. Here are some examples:
- Demanded money that was not owed.
- Murdered a horse.
- Busted a barn door with the tractor.
- Busted the tractor transmission by speed-shifting.
- Stole two lawn mowers.
- Stole a truck.
- Stole some tires.
- Ruined a horse's training.
- Was hired to feed while we were away, but didn't.
- Sold our animals while we were away.
- Didn't show up for scheduled work.
Simply don't be any of those, but instead do a sparklingly brilliant job taking care of the horses and anything else that needs attention and you will later be able to name your own price for some other kind of work that is less back-breaking.