I battle a similar problem, riding "toe down" when not really attempting to ankle. I've been riding since the 1970s, and only came to realize this when getting fitted on a road bike back in 2009. She (the fitter) says:

"You need to level your foot."

Me: "What?"

Her: "Your foot is toe down. Bring it to level when you pedal at the lowest part of the stroke."

Me (thinking): "Listen, girlie, I've been riding like this since your parents were in diapers."
Me: "Oh. Okay. I'll try"

Her: "That's better. Try to stay aware of this until you correct it."

And that was the gist of it. Since that day, I'm conscious of foot positioning from time to time and try to relax my foot at the neutral position of the stroke(s). It'll take concentration, but you can do it, too. Good luck.