I think you are foiling too high - AND, just need some more time on the water (I know you from DK so have an idea of your time).
And some foils, are trimmed so they can only be ridden "fast" with load on the front foot - meaning, the faster you go (downwind that is), the more you have to load your front foot.
Other foils are "neutral" (mine is), meaning you can accelerate in speed without having to push further with your front nor your back foot.
This is only personal experience, what one prefer I think
So if you have one of these foils, that needs extra load on the front foot downwind at higher speeds, it takes a bit more "getting used to" of course.
Also, my experience, is that you feel that downwind at really high speeds is almost impossible after the first half year only - but during time you can ride at insane speeds and still feel "comfy".
Just a matter of time and muscle memory, nothing else - so just wait and it'll come