How can she be nicely collected and not pulled down by the hands, if that horse is - as you admit - not working with its hindquarters properly. That doesn't make sense at all. And you can clearly see the open mouth.
@HorsesAreForever: You can't really tell from a picture, so why don't put in a video? And by the way, 'head down' isn't everything for dressage, especially in competitions for beginners. Improve your seat, first! That's so important and gets forgotten all the time... By the way, the horse I used to ride was just as hard in the mouth was you described your horse (if not worse..) - try to start bending her and reducing your rein impulses, that helps alot. I don't know what you call it in english, but if she lowers her head only a few milimeters and gets lighter in the hand, you have to assure her that she's doing the right thing by giving her a little rein. Never forget riding ahead! By which I mean that she shouldn't lose her tact (that's what you call it in english?) She shouldn't be fast, but energetic, means using her hind quarters. In trot her hind hoofs should be in or over the prints of the front hoofs without becoming too fast. She should learn to stretch herself foreward/downwars before beeing collected.
That for a start. I would recommend you a good book, but I don't know any english ones.