I'm wondering if over the years she's been allowed to use the lead of her choice, maybe for public trail rides. As long as a horse will follow the one in front and do as the others, there is often little training done. Here's something to try. As soon as she takes that lead, slow her down and walk half the arena then ask again. Since horses like to conserve their strength asking her to walk will be seen as a reward. You could be trying to break an old habit and it won't be easy. Be consistant. Don't slow to the trot, but always the walk. If you are consistant she will realize if she does this, she gets to walk. The gradually up it to two strides then three. Also allow her time to build muscles. She may be weaker on her near hind as that is the leg she drives off with.