I do think that slowing it down at first will help actually. That is, IF you need to do it at the gallop, I can't really tell from your post whether you want your horse going that fast or not. But at a slower speed at first anyway, you'll get used to the feel of the movement, then if you need to speed it up do so gradually and it'll be easier.
The other thing to remember is make sure you're not leaning into the turn too much because that will unseat you and unbalance your horse. If we're not addressing what you're describing, please explain a little more clearly, your post is a little difficult to understand. For example: what about staying in the saddle is hard? Are you falling off the side, or bouncing a lot?
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.