Bend your knees man.
Try doing grab turns to improve that
Andd grab turns look so dumb if your watching it, but it just might work for yah
When your on your heel edge, grab your toe edge with your hands
And when your on your toe edge, your supposed to grab your heel edge but its pretty much impossible to do, so just grab onto your highbacks.
I'll go with everyone else and say bend your knees more and really get down over your board. If your knees are bent and over your board you should be holding your edge really well and making a single line. What board and size are you?