How can I keep my feet together in a back hand spring?

I've been cheering for quite awhile now, and my tumbling skills have been off an on. I'm a fairly decent tumbler, lots of power and stuff, but its still my weak spot. I want to say, back in Feb I noticed this problem. In my back hand spring, I tend to pop my left foot out, causing me to go crooked, and its now become a habit. I'm extremely frustrated with this, and I'm squeezing my ankles and my together, hoping something would work, but it doesn't! I'm at a loss, and so are my Coaches. How can I keep my feet together, right before I actually do the back hand spring? I happens right when I go to push off!