Every time you go out, practice riding without
stirrups for part of your ride. Just take your feet out, do little ankle circles, point your toes down then up and forward, and get a better feel for the way the horse is moving beneath you. Then, without looking down or leaning forward, put your feet back in the stirrups.
If you get to the point where you rely on your stirrups too much, then if you ever do lose one (foot slides out, horse stumbles or spooks, etc) you will still be able to maintain your balance and composure until you can get it back again.
Sometimes a fall is just unavoidable. It happens. But developing good seat and balance independent of the tack you are using will help you KEEP your seat when you find yourself in a rough spot.