"I am not myself" is clearly wrong if "I" and "myself" are the same, just as "dead" cannot not be "dead".

"Dead"'s (or anything's) primary essential property is that it is itself. Dead is Dead.

This is similar to the definition that statements are true xor false (excluding paradoxes). They cannot be true and false, nor can the be neither true or false.

Dead is dead. The question is "is it all dead, or is only the material part dead while the mind/spirit lives on?" This one is harder to prove. I willn't try, here.

Dead is dead, and nothing but. This is the simplest possible assumption, and so in the absence of any evidence from beyond the grave of life after death it is most likely to be correct.

To say that this can't be so because you don't want it to be so (maybe because it scares you or something) is to assume that wanting it to be so in itself makes it so. Sorry. Learn to live with the immeasurably vast indifference of the universe to your hopes, fears and desires.