I have a very forward mare, especially when we're coming down from faster gaits.
What really helps her is to not tense up with my reins, but follow them. I also try to really relax in my saddle, keep an even, slow breathing pattern, and set the pace with my own body by replicating a slower beat with my lower body, while really engaging my thighs and core so that my lower legs stay in a proper place and don't encourage her to go forward.
I had the problem of bracing with my upper body, which led to a jigging horse! I wasn't setting a slower pace with my body, which was telling her to go forward while my hands and arms were telling her to stop. All that energy I was putting into her was causing her to go up and down since she couldn't go forward, leading to jigging!