Hi David,
I think you could do worse than to get an assessment with someone fresh, from outside your normal airfield/school.
A few people are around (Irv Lee at Popham and Derek Davidson [sp?] ) spring to mind. I have no experience with either but their names have popped up here and on other forums when similar situations have arisen.
If you are that close to the skills test they should be able, in a relatively short time, to see if you are able to cut the mustard, and where (if any) you have any weaknesses.
Nothing worse than a crappy instructor spoiling your day and to leave now would be very frustrating to you for a very long time.
Unfortunately not all those with an instructor's ticket are gifted with an awful lot of teaching talent and patient but you have to guard yourself from being a victim of that.
To be a good pilot you have to possess a modicum of assertiveness, commitment and perseverance. Surely attributes you have as you have come such a long way already, let these qualities help you to achieve your goal!
Have fun in pursuing your ambition, remember the sky is the limit!
FD