Join Date: Jan 2012
Location: Cow Town, Alberta
First, I'd like to say I've been in a very similar situation myself.
First is the stopping problem. While you are riding, try to focus less on your hands and more on your body language, and believe it or not, your eyes. Make sure you plant your body down into the seat of the saddle and stop riding his gait. This will be a definite sign for him to at least slow down. I'm guess you know the phrase "look where you want to go"? Well, same goes for stopping. To halt, stop looking where you want to go, and focus on his head/pole area instead.
The Cow dilema, unfortunately, I don't have much advice for this, mainly because I'm having many of the same problems you do, but my main piece of advice would simply be to follow the cow's movement yourself, while you're riding, so that possibly, he will pick up on what you are trying to teach him, and possibly do it himself. Oh, and unlike what Lynn said, I wouldn't let him trot with the cow, or lope with the cow. The objective is to keep the cows moving slowly I.e. Walking. :)
As for Gathering, I don't have much experience with it, but I would say, just keep trying, maybe more and more gradually, until you get to a point that you are happy with, maybe an even slower trot ect. Just keep at it... Sorry that's all I have :/