1. "One major feature that raises human beings above the kingdom of animals is that human beings have rationality"

Wrong, if animals do not have the ability, they'd be long extinct.

2. "However, such a disease as far as I know have never been documented, which makes it extremely impossible that I am suffering from one"

There is always the first. So it is not a key point.

3. "On the other hand, if I did have such an incredibly impossible disease, I only have symptoms in a certain period of time, at certain geographical locations, and only a specific aspect of my mentality is involved with the rest of it functioning normally? Not a single chance"

Why not a single chance? It could only happen on certain occasions whatever the reason behind it, it does not have to be regular or consistent.

4. He is the one making self assessment, even if he is a doctor, he is not in a possible to make those observations and come to those conclusions.