so you're having trouble on heelside turns. usually 1 or the other gives alot of people problems.
heelsides are also my problem. what i do when im going onto my heelside edge is keep my
weight centered, and flex my front toe up, which twists the center of the board to put the front
heelside edge into effect. then you have to just lift up your back toe and wham, you're on heelside!
this all has to be as fluid of a motion as it can be, or you'll catch an edge, or start flat spinning,
which is what you're doing now.
the trick is to keep your weight centered in these. too far forward with your weight and you'll be
whipped over, but thats pretty far hanging out forward. leaning back gives you the opposite effect
and causes you to get into the stop position on heelside, and spin you around.
you'll have to find your balance point for heelsides since they're your problem, and im guessing
your skill in toeside results from skateboarding..thats where i got mine.
heres some advice i actually got from a riding instructor.
position for toeside - pee like a boy
position for heelside - pee like a girl
a few of my friends learned to ride on this advice alone. maybe it'll work for you too i dont know.
some others will chime in with their advice, take all you can get, everyone has a different way
of learning, one of the methods surely will work.