You are doing nothing wrong. From the sounds of it, you are edging through the wake as you are supposed to, although the handle could be high. Keep the handle low (belly button or so) and this could fix everything. As you continue to gain consistency with your edge into the wake you will no longer notice the board feeling like it is going out behind you. However, if the board going out under you is extreme simply relax your cut to the wake a bit and that should resolve your concerns. Any time you feel the board going out beneath you just push the handle down and this should get you feet back under you. Summary: Rope low, edge through the wake and land on other side. Keep riding hard and good luck.