Your stirrups may be too long. If they're past your ankles when you stretch your legs down, they're far too long. Also, no stirrup riding where you try and keep your heels down and toes up without stirrups really help! Try to focus also on keeping your leg still and your weight not on the stirrup, but in your heels. When you post, raise and lower from your leg, which will teach your legs to be still. Hope I could help!
To start off you should put all your weight into your heels. Stand up in the saddle and when you sit back down keep your heels in place because when you stand up all your weight goes into your heels. You can also do stretches on your stairs or steps. Put thw Ball and toes of your foot on the stairs and attetch your heels all the way down. Hope this Helped!😊
It's nothing we can help with, the only thing to do is to train! Try to stand up for a while. It will stretch your foot a bit and will make it easier to keep it down.
Also try to take your foot a bit forward! It will come with time! Had the same problem when I was little :)