No worries, diagnosis isn't a requirement. Forums are just meant to be "best fit" for whatever you're discussing so that people with similar issues are more likely to see it.

As for your dad, I wish I could offer something helpful.

All I can say is that the school's instructor has likely seen it all and is way more qualified to judge your abilities. Try to believe in his opinion. It's bound to be more objective than a family member's.

I went through something similar years ago. My dad and brother were overly critical and made me very anxious and doubtful. For what it's worth, it's a fairly well known phenomena that parents often make terrible driving teachers. I ended up paying for extra lessons.