Going over pointe shoe box because of arch and ankle?

I've been on pointe for almost a year now. My teachers says I have a pretty arch and need to learn how to controll it. My right ankle has gotten a lot stronger, but the left is weak. I have a hard time staying on the box because my arch allows my foot to be more mobile and flexible. I literally have to use all my strength to not go over the box without the barre. Do you have any ideas on excersises I could do to strengthen my left foot?