you should be popping off the lip more, pushing yourself up and forward a bit. Then spin. It sounds like you're spinning off the lip.
dont jump to early (thats the problem i had)
and lean forward a little bit more
landing backseat = shinbang
^ thanks, i was going to say that, but you took care of it already plus karma!
What i think for learning spins is do it out of a quarterpipe onto a deck, that way you are not moving orizontally very fast and you wont catch an edge and die, your feet should do the slightest pizza on the lip so you are throwing off of your edge, it gives you ALL of your rotation, look to the side as far as you can for a landing, as soon as you see it, open up like a mother fucker! done...