Sounds to me like your stirrups are too long- try putting them up a hole or two, and make sure you're not shoving your heels down instead of actually sinking your weight down. Shoving your heels down can pull your toes up too far, which results in losing the stirrup.
I used to have the same problem, I figured out what helped me was pushing down on my stirrups almost an exaggerated amount. Over time my feet stayed in the stirrup and I have a strong lower leg. Try just really wrapping your legs around the horse and pushing down at the same time. Try it in walk, trot and canter and see how you get on. I also found riding in the two-point position helped too because I was forced to keep my feet in the stirrups and legs strong. Hope this helped xx
Ive been riding without stirrups all summer and i find its helped me build my leg and lower abdominal muscles.
Try to think that you are putting just a bit of pressure on the stirrups, and try not to swing your leg back unless you need to :)