Hill work and a lot of trotting!! Ground poles can help too
Lots of trotting in a frame!
I had to do this with my 16.3hh warmblood as well when I first got him.
Do ALOT of long and low, by ALOT I mean everyday for like 10-15 minutes. At first it will feel really weird for her so be very patience.
Do some cavaleties
Do a serpientine with her head set in the correct position and make her almost over bend when you turn her to the next pole.
I lunge my mare x3 days a week in a Pessoa lunging system (Long and low setting)
And find it helped her build and makes your riding work easier as she will get used to carrying herself instead of leaning on you for balance.
Working on the bit and round