Okay, my gelding had a real problem with being heavy when cantering. He was totally pulling me down and riding was no fun. Over a course of about four or five months of riding twice a week, sometimes more, he got only a little better. What kind of bit are you riding your mare in? I rode my gelding in a jointed snaffle for the longest time...my trainer gave me a pelham to try on my horse because he was also very hardmouthed. It was like day and night. Rusty lightened up, and while he still has a long way to go, I can actually stand riding him because he isn't pulling me down. I'm not saying try a pelham bit or anything, but don't be afraid to try different pieces of equipment if it's really giving you a problem that is interfering with your ability to fix the problem (ie-not wanting to ride because you always get pulled out of the saddle and nothing seems to be working). Also, in my situation, Rusty had been a western horse, used to a curb and taught to drop his head and get heavy (hence why I ride English!).
Also, I've heard that lots of transitions work and so does lateral work. Also, going for a trail ride and riding up and down hills helps build hind end muscles. When she gets heavy, you can "pop" her with your inside rein (quickly pop it up to her ears and release RIGHT AWAY when her head comes up). Also, make sure not to drop your hands.
I totally sympathize with your situation. Heavy horses are really hard to retrain.