Are you riding with a dressage length of stirrup or jumper? Think of opening your thighs and putting your weight down into your ankles. Toes up. When you're doing it don't go against the horse, absorb their movement. Your heels should go down every stride when you're absorbing the energy. Try watching a YourRidingSucess video on YouTube that should help a lot she has great tips for sitting trot!
I had the same issue and what I did was really sink all my weight into my heels. I try to keep my bum deep and feel as though someone has glued me to my seat. I also disripute all my weight between my shoulders but also my hips. It really eases the jostling, and If you can find the horses tempo, that really helps.🙃💙
Try to work more on your sit, do sitting trot first without stirrups, focus on keeping a good sit across your arena. Once you add the stirrups try to stretch your legs as far down as you can while keeping your heels down. Also try to notice if you’re holding on more with your knees than your lower leg. I struggle with this as well, this is what my trainer tells me to do and it has reall worked for me 😊 hope this helps!
Try as hard as you can to keep your heels down and keep your weight in your stirrups. This should help you a little more with keeping your feet in the stirrups. Also make sure your stirrups are on the ball of your foot and not your toe. If you are an eastern rider and you are used to having shortish stirrups, then check that your stirrups are at the right length and not too long. I made that mistake today by having my stirrups two holes longer than I should have had them. If you’re having a lesson and you discover half way through that your stirrups are not the right length, change them to be the right length so it improved your riding and you ride the best you can