Thank you so much for replying. So I think I know where I might have gone wrong...
I started off with a couple of individual lessons then signed up to Take up the reigns (adult beginners) - but this didn't get any other sign ups so they put me with Take back the reigns (adult returners). I hadn't appreciated how much of an advantage this would put the others to me.
The lunge lesson was on a horse I don't particularly like riding (she has a wiggly head which puts me off!). My instructor is very firm about everyone needing to learn to ride every horse, rather than having favourites - as she believes this makes you a real 'rider'. Which is fine - but I'm very sorry obviously nowhere near a real rider. I'm a 42 yr old woman wanting to keep fit and have some fun. In these circumstances, is it ok to stick with horses I know I like?
He wasn't meant to canter - we were trotting around the school in a lesson with 6 others when he got confused and decided to come off the path to follow one of the other horses (who might have been cantering). The change of direction and speed quite literally threw me! I actually felt one of my legs come up so panicked and thought it was better to throw myself off cleanly, than risk being dragged). I have since realised the more sensible reaction would have been to cling on for dear life 
Mentally I really want to go back, but yesterday I found the physical sensation of riding (trotting, I was finewalking) very different - before the fall I found it exhilarating and thrilling, yesterday it felt like nauseating fear. This could be muscle memory? Do you think if I keep going but take it slowly this might wear off?
I've asked if I can transfer the rest of my sessions to DD for pony club - then I might try an individual lesson with a horse and instructor I prefer. Even if that means just walking around the school for hours practicing my balance.