Circles, circles, circles.
Instead of pulling back to slow her down, turn her in a circle- this will get the pressure off her face and she will find she likes straightness better.
If she is walking along consistently and speeds up- circle her.
If she breaks into a trot- circle her.
Keep circling until she breaks back down to the gait/speed you want and allow her to move straight- rinse and repeat ;)
It doesn't need to be a small, tight barrel turn but a 10 ft circle would do just fine.
Start off at a walk, hold the reins on the buckle, or nice and loose so that there is a loop in the reins. If you horse breaks forward into a faster gait, pull her nose around to your toe and let her circle until she stops and softens. Then throw your rein away, give her a minute to stand as a reward, give her a scratch, then continue.
The horse will soon learn that it cannot run off and if it travels at the pace you decide it is rewarded with a loose rein.
You will pob. Also find, once you have established this the horse will respond much better to an ordinary halt (using both reins) as well. Lateral flexion is the key to vertical flexion!