Fell off for the first time, what can I do?

Today I fell for the first time, 30F, been riding on and off most of my life. I'm learning to canter on a western pleasure horse and he's a dream!

I have a bad habit of putting my leg behind my body and I ended up bouncing too much and essentially launched myself off him. Nothing broke or bleeding, took a little walk and got back on, thanks to my trainer. She says I have a great seat but I'm still not used to cantering fully.

Is there anything I can do at home to help train keeping my leg underneath myself on an exercise ball or anything to somehow simulate a moving horse? I only ride once a week so if there's anything I can do outside of that that'd be great!