Reading David's posts, I am of two minds:
(1) I too have had two or three bad experiences with "instructors" who misconceived their role and wished to play "examiners" ... a semi-adversarial position that does not make for effective learning.
(2) On the other hand, it sounds as though this 3rd instructor was specifically asked to ensure that David was ready for his skills test; so I don't think that he can be blamed for coming on a bit strong and asking some difficult questions. Most decent flying schools give a pretty gruelling mock exam, on the theory that if the student can muddle his way through that, he should have no difficulty on the real thing. Of course, this does not justify asking all sorts of irrelvant questions about obscure trivia that the student can't reasonably be expected to know, or demanding answers in a sneering, contemputous manner (not sure whether either of those problems arose here).
I was thinking about all the money I've spent only to get treated like an idiot. And all the other things I could have spent it on, which would have made me feel a damn sight better than I did then! So the idea of giving up seemed perfectly reasonable. I still feel that way to some extent, although I can also see that it looks stupid to give up at the last hurdle.
I agree with Monocock: only David can decide whether he wants to continue, without consideration to such issues of being a 'quitter', etc. Flying is NOT for everyone, and a fair amount of sacrifices (financial, time, etc.) are required (post-license included). If one is truly hooked, than these irritants are minor; but no one should fault David if he ultimately decides "I don't need this, I'd get more fun playing golf".
I don't think that it would be stupid to give up at the last hurdle, IF David honestly feels that flying is not for him. He shouldn't be influenced by others' opinions, or by all the time and money that he has already sunk in (that's all water under the bridge, and there's no sense falling into a 'previous investment trap'; see further
Harry Browne's freedom principles ). David, think about what
you want in the future, and act accordingly.