How do you ride your corners is the first thing I need to know? Do you look around the corner to where you're going? Do you put weight on your inside seat bone and turn with an inside rein? These things, although many people will shout 'inside rein, weight inside seatbone look around the corner, inside leg inside leg inside leg!!', this won't necessarily get you a good corner ;)
Try something for me next time you ride. Have someone on the ground to watch you so they can tell you when things happen.
Ride to your corner, look around the corner, weight your inside seat bone and pull your inside rein to get around the corner. Ask your friend what happened to your horse's legs, and how many metres out of the corner you were. I can tell you right now, if you pull on the inside rein and weight your inside seat bone to turn around the corner, your horse's back legs will cross over and he will cut in on the corner. You do NOT want crossed hind legs, ever! (of course the lateral exceptions, half pass, leg yield etc etc etc.) but in corners, no crossing of the hind legs is to be seen.
Now try this.
Ride into your corner and keep looking straight ahead. Don't look into the corner. As you reach the corner, simply move both of your hands towards the inside (without crossing your outside rein over) so that you're giving your outside rein. Think of it as turning a wheelbarrow. You can't turn a wheelbarrow by pulling on the inside handle can you? You have to push it around with the outside handle. Same deal with turning a corner on a horse! You don't want to throw your outside rein away, but you do need to give it enough to allow the bend to come through the corner without crossing the hind legs.
Now ask your friend if you got crossed hind legs, and how far into the corner you rode?
I've done this exercise with a couple of girls that I've taught and teach now, and every time, every one of them has ridden their corners within a metre of the corner. In grand prix dressage, you need to ride your corners 1m in, and these girls are riding their corners like you should in Grand Prix :)
To make it a bit easier, stand in the corner of the arena, take one big step into the middle of the arena (1m), then another step (makes it 2m) and put a cone where your foot is. Ride between the cone and the corner!!