Most of the time you can see when a student is getting fed up with you. Time to sit back,fly somewhere interesting locally (even just fly the student somewhere yourself) and remind yourselves that flying is fun. Very, very occassionally I will raise my voice or change my laid back attitude. The ONLY time to do this is to reinforce a point that the student has done something unsafe. So unsafe it could kill you. That is the only time at which you want the student to remember having a bad time in the aircraft. Maybe David is truly cr@p and dangerous, but I think someone would have noticed by now. I am sure most instructors have had to tell someone that they will never fly solo. Then again maybe not.
Another point is that as you get older you learn less quickly. Young instructors don't realise this. When I was 19 I soloed in 8 hours and gained my PPL in 17 (yes 17) flying days. Some of my older students have taken 30 hours to solo, not coz they are crap, but because they are 55 years old and only turn up to fly once a week - and not coz I am a cr@p instructor. Then again maybe I am.
David, if you want to fly then take a break and come back to another school and another instructor, and remember why you wanted to do it in the first place.