When you say flexing do you mean vertical flexion or horizontal? I'm assuming you mean vertical, because you want the western pleasure headset.
First of all, I would recommend you do a lot of stopping and circling to get her to go the speed you want. Also, you want to master horizontal flexion before vertical flexion. Horizontal flexion is the stopping power and vertical flexion is forward power. You wouldn't want to put the extra gas on before you install the break would you?
Because you say she moves with her head up high, I would say you need to work on horizontal flexion. This will calm her down and make her easier to stop. Also, everytime she goes faster than you want, you can flex her to the side (one rein stop) and she'll quickly learn to go the gait you want her to.
Once she's mastered horizontal flexion then you can move onto vertical flexion. Vertical flexion will give you the headset you want and the nice smooth slow gaits if done right.
Hopefully that helped. Feel free to ask questions of PM me
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