I don't know about you, but before every flight I take a sample of fuel. I then go to the local weather station where there is sure to be a big 30 in. tall mercury barometer. I insert the fuel sample into the bottom of the barometer and let it float to the top of the mercury. The fuel equilibrates with its vapor, generating pressure at the top of the mercury and pushing it down. I then compare the vapor pressure against the NIST fluid database to properly identify the hydrocarbon mixture.
It sounds like your DPE and your CFI (and in turn your school, as everyone who works there would be influenced by whatever your CFI is spouting at the water cooler) are not giving you a fair shot. They may not be reacting to anything you actually say or do, but to some conflict of personality that for whatever reason is difficult for them to work past, hiding the real good you are doing. There are many many experienced instructors in the bay area, and I'm sure after you get this one wrapped up you'll find a much better one.