staying soft, rating, pivot foot, reining horse need help.?

6 year old breeding stock paint, been ridding him for about 3 years now. I want to show him someday, in reining. Big issue I have always had with him is that he is forward, what I mean by this is he is always trying to go faster and faster. Everyday when I ride I try to get him soft, He gives very nice at the walk and trot, I can also move his shoulders and hips where ever I please and he still stays soft and keeps a certain pace. However when loping he looses himself, especially when he is fresh if I put any pressure on him like ask for his face or ask for his shoulder up he just speeds up. So typically I just circle him up and let him work, he does get soft and feels really round in the circle but as soon as I let him go out straight, again he wants to charge off and his head comes up. I have tried stopping him and backing him up, I circle him, I'm just so frustrated. Also I have always ridden him in a snaffle, I try to keep him as light and responsive as possible, just when we get loping its like I can't get through to him.