I posted this response to another poster that had similar questions. I just copied it and pasted it here for you. It really helped me. I agree that you should be cantering well before you get to the jump, but you also need to teach her listen to you.
I can't jump except in my weekly lesson (which is fine with me because I don't want to jump my horse too much), so I had to figure out how to get her to listen to me while jumping without actually jumping. Here is what I did:
I would take a line down that was heading towards the gate (because that is where my horse likes to rush). Leave one of the poles between the standards, but laying on the ground so the horse can just go right over it. Then I worked on getting her to go down that line at the speed I wanted her to. First, we just walked it. I would walk over the first one, slow down a little, then walk over the second one at the same pace. Once I felt we were good I began trotting over the first one, bring her down to the walk and walk over the second one. Building up to trotting through the line completely at one same pace that I liked. Then I moved on to cantering, bringing her to the walk...cantering and bringing her to the trot, etc. Until we finally were cantering the line at a good, normal speed. This took several times before we actually cantered the line completely but it has helped.