skater1964 has great advice! :agree: You've gotta push your hips forward to counter the weight of your head and leg going backwards. And it's really helpful to try the position at the boards.
Face the boards and hold on with both hands. Back up a little so your spinning foot is about half a boot length away and then press your hips forwards until they touch the boards. It's important to keep square - keep your foot pointing perpendicular to the boards and your hips should meet parallel. If that distance is easy, move you foot farther from the boards and try again. As you push your hips forwards you will notice that you have to lean backwards to maintain balance.
Next, try the spin with your hands on your hips. Start the spin, then as you bring your freeleg backwards, push your hips forwards at the same time. Keep your stomach muscles strong as you're starting to lean backwards - it will help keep your position strong.
It's quite disorienting when you do spins with your head in a new position (like upside down) but you'll get used to the feeling and your spin will get better. I have feet that pronate as well and it doesn't affect my layback at all.