How to not fall off on a Cantering gait?

I just started riding a new horse ( She rox and is so sweet) and I ride english. I have got the Posting trot and diagonal down, but i'm a little shaky when it comes to cantering. When I canter I feel the need to grab on to something, so I grab the pommel (front part of saddle) . Could I fall forward? Also are there any more ways to break in riding boots than walking up stairs and riding?