I have an older gelding and ride lots of stubborn horses who love to stick their nose in the air and keep their back stiff and hollow. Basically a ton of RELAXED long and low will help a lot! Especially if you can get her to do it going up and down hills with her hind end reasonably engaged. Use your seat (and press and release with your legs if she is lazy) to push her forward into soft hands and elbows so she reaches for the bit and then allow her to stretch down and bring her back up. Don’t let her leave her butt in the dust, keep it with you and try to get her hind legs stepping up and under as much as you can. Ground stretches may make riding easier. While you groom her you can take treats and get her to follow them around to each side of her barrel. Start behind her shoulder and as she gets used to it and stretched out increase towards her flanks. You can also bring a treat down between her front legs to get her to lift he back and stretch over her topline.
-Hope this helps!
The triple threat of riding = EVENTING! 😉