Oh and do this over a series of poles and slowly ride which should help,also I think hill work on a loose rein should help her start using her back more which will directly make her use her neck right too :) hope this helps
Ride her on a looser rein with leg to encourage her to stretch over her back and step under herself more try be as supple and light on her back as you can be and the spine and neck move as one and if you are too heavy on her Bach she will hollow through.
it being short wouldn't / shouldn't do that. her neck tying in high will cause an elevated headset. get good impulsion & open your hands wide. stay soft in the elbows & help guide her to stretch. it takes muscle so dont expect results immediately. pressure and release is key