You may need to get wider stirrups or ride in a narrower boot. If your feet are slipping right through then getting stuck then raise your stirrups a hole or do some work without stirrups to get your leg longer and more relaxed. Really work on your leg position, working on getting a solid, still lower leg, then your feet should stop moving about in the stirrups.

You may need to get wider stirrups( they are sold in different widths) or get the bent leg stirrups, so your foot doesn't get stuck.