Okay, so, I have a six year old 16.2hh Quarter Horse gelding...he's absolutley the sweetest horse I've ever met, such a puppy dog in disguise. He's incredibly smart, but we have issues at the canter - he picks up his left lead perfectly, but we have major issues on the right lead. It's almost like a fight for him to pick it up, and a lot of the time he'll crossfire and start bucking. When he does get it, it's kind of rough and choppy, whilst the left lead is nice and smooth. And, when he gets his right lead, he just likes to go in...and keep going in...he won't stay on the rail if I ask him to, or if I ask him to turn he just falls on his shoulder. Also, about a year and a half ago he injured his left hind - a good six-inch gash all the way down to the bone, but it's right below his hock and right above his fetlock. His leg is a little thicker than the other one, and there's quite a scar, but he's otherwise sound. I've had him for about nine months, and we still have issues even getting into the canter - he just trots faster and faster until he speeds into it, and he's really tense about it. I think that I may be part of the problem - I've only been riding for three years. although I have the help of a trainer that's been traning pretty much all his life [he's taken horses to World shows and all that jazz.]
However when we do canter, he collects nicely and doesn't speed off or anything. He even has problems when we're just in the round pen and I ask him to canter. It might take one or two tries until he gets on the right lead without crossfiring, and when he does his head is usually stuck up in the air and he speeds around like a bullet. He's only been lame on his previously injured leg once, and that was from stepping on a rock. Any suggestions?!
Edit: Pictures of the silly boy.

My pretty boy and I this summer.

Prancing for the lay-deez.