Ladymoonlight...like you said, you weren't confident about the work and you were not strong. In my opinion, a lack of confidence with what you're dancing will always have a knock on effect to your performance!!
From what you've said..I don't think it's the fact that you can't do the work!!
If you work hard and practice over and over again these exercises, you will be as confident with this as you were with your grades 4, 5 & 6 !!
I totally sympathise with you, I've been in the same position!!
No-one likes to do badly in exams especially when you are so passionate about what you're doing and when you've acheived such good marks in your other examinations but, I find, once the disappointment has passed you will have a huge determination to get over this small obstacle, so turn a negative into a positive and make it a new goal, everyone needs something to aim for or life would be boring
It doesn't matter about your age in my opinion...so what if you're in your late 30's before you're qualified?
Think of the experience & knowledge you will be able to pass on to your students about exams, techniques, tips etc!!
Anyway, just my opinion...hoped it helped a little
Sorry it was so long