You can get string and tie your stirup to your girth
I have always been taught that if your horse disappeared from underneath you then you would land in a crouched position without falling over. This has always helped with me.
Hey Alicia! I would agree with everyone maybe put your stirrups down a hole and a lot of no stirrup work! You just need to get your lower leg/core stronger! Hope this helps!
Have you checked your stirrup length? This can happen if your stirrups are too long sometimes. Otherwise it's simply a case of doing some stirrup less work to improve your core stability and to stop you from relying on your feet for balance.
Jumping without stirrups can also help. This is because you are relying on your leg to hold yourself up, so physically it can't swing back. You will just have to ride without stirrups a lot.
Just focus on using your core to anchor yourself, and make sure to keep your leg on before the jump. If your leg isnt on in your approach, it is more likely to move over the jump. Bend at your hips, to throw your weight into your heels