Some people don't have a very flexible ankle joint, which can be worked on to a point (not to the point of pain). To get your ankle flexing better you can try standing with a phone book (start maybe only 1" thick) with the ball of your foot on the book, and the heel of your foot resting on the floor). Also when doing this or riding think of the weight going down through your heels, not toes. Otherwise you are off balance (besides the obvious danger of foot slipping into stirrup). Also try an exercise where you place feet flat on floor, shoulder width apart, think weight down through heels, then let your butt sink down till it is nearly on the floor, place each elbow against the inside of your knee and press them slightly out, keep chest and head up, back straight. This is going to seem nearly impossible and really weird at first, but aim to keep feet as flat as possible. With time you will get it and it should help a lot. At first you can only do this for a short time, then stand and relax, release muscles, but they will get better at it over the course of a couple of weeks. Also check whether your boots are a bit stiff in the ankle, makes it harder to flex, use a little leather oil treatment to soften up.  And in your normal daily footwear, the flatter your heel the better. If you can find "Roots" shoes, they used to be designed to get your ankle flexing more, I don't know if they are still around or made for that. (Dating myself here). Hope this helps.