How do i slow her down ?!?

I have a 10 year old TB mare who has raced and hunted and obviously not been taught correctly. She goes fab on flat work and really enjoys jumping and clears big jumps, but its the speed she goes into them at !! If im doing a course it takes a few jumps for her to settle down, she is just a really excited horse and thinks she needs to go full belt into jumps ?? I only have a snaffle in her mouth at the moment and i have tried a dutch gag but it doesnt seem to make any difference and i dont want to hurt her mounth. Any ideas what kind of grid work i should be doing or where i can get good examples of layouts of grid work ? Thanks xx