I can't really speak for a show scenario dressage test, but I can say that in my general experience its those "crushing" critiques/scores/grades that help me the most sometimes. Yeah, it stings a little at first, especially if you aren't used to it, but it does teach you.
One of my favorite college profs has never given me an A on an exam - papers, quizzes and homework, but never an exam. I've studied my guts out for every one, every semester for over two years and still the A is elusive... but I always leave his final feeling like I've gotten my money's worth out of the course, and like I know a heckuva lot more than I did at the beginning of the semester.
One of the things that drew me toward dressage is that the emphasis goes back to self-improvement for the horse/rider team, and the judging style reflects that.
I strongly encourage you to look past the disappointment, study your score card, and take it as another learning experience and opportunity. It isn't a matter of acceptable/unacceptable scores, it's about improving you and your mount at the right pace. Step back, find the holes and gaps that the judge noticed, fill them, and try again.