How do I keep the stirrup on the balls of my feet without it slipping back all the time while I’m trying to kick?

Keep heels down may be an easy fix
Shorten your stirrups as it sounds like they might be too long for you if your feet slip that easily. If they are a suitable height I would suggest doing some non-stirrip work to stretch your inside leg and then you will feel that your stirrups are easier for your feet to reach. I have this problem when I do a lot of dressage I forget to put weight down my legs and my stirrups just fly way.
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