1. I just watched her video from Dream Horse. Her under-neck is pretty well muscled indicating to me that she is ridden with her back hollow often and is accustomed to bracing. If this is the case, it is a training issue and will take time to correct. I suggest working on riding her in a "round and down" frame while warming up, ALWAYS keeping her back soft and supple and lots of impulsion behind really pushing her into the contact. Make sure your wrists are supple and arms are nice and supple as well. Also, she is on the fore often in the video. I would work on lots of upward/downward walk/trot, trot/walk transitions eventually leading up to canter work (lots of transitions in canter too) when you can successfully ask her to transition upward/downward without hollowing/tensing in the walk and trot then move up to canter.
2. When were her teeth floated last?
Also, make sure she is not behind your leg. She looks behind in the video, until she is jumping lol