First of all, whatever you do – DON’T USE A STANDING MARTINGALE! They gain muscle in the neck in all the places you do not want. They gain the strength to pull up or “giraffe” on you.
Try this: when you warm up, flex her head. This will stretch her neck for the workout she’s about to get. Then, at the W/T/C, put your hands UP, and lightly seasaw the reins. This will teach her to curve into the bridle, instead of diving into it (as seen if you ask her to collect with your hands down).
If this doesn’t work as quickly as you would like, I suggest finding a good Running Martingale. It helps to start out with one that has elastic, but if you can’t find one for a good price, ones without elastic work just as well. SiDE reins/draw reins/etc. are good for lunging, but not riding. In fact, before you embark on using a martingale, try lunging with side reins.