congrats! for getting yourself hooked
I've watched your vid all the way
I think it's cause your knees are bent
your legs are supposed to be straight and you're supposed to look like you're leaning on an invisible wall and should be standing diagonally against the sail to get that sail up and steady
just sharing what I've learned when I went wind surfing a couple of times in Batangas
it's not easy and you'll get sore wings the next day
good luck
you look like you're having fun though :)
take care !