Your mind isn't empty, in fact a SPECT scan would probably show that it certain parts of your brain are more active than what they should be under normal circumstances.
I suffered from the same blank / foggy brain symptoms. In fact I was so worried about the functioning of my mind, that I felt compelled to take a SPECT test (maps the blood flow of the brain) in an attempt to determine what was working correctly, and what wasn't.
The SPECT results were nothing short of astonishing. After suffering with DP and its fog for seven years, I had developed a fear that either drug use, medication or previous concussions had resulted in an organic brain injury responsible for my woes. The SPECT results disproved all of my fears by revealing that I was suffering from an overactive anterior cingulate gyrus and an over active deep limbic system. My mind felt blank as yours does, yet it was actually hugely active in a non-focused ?noisy? kind of way.
These two systems are commonly associated with anxiety disorders. The elevated activity in these two areas of the brain symbolized an acute anxiety response. A response that was shutting down the executive centers of my brain and making it hard to think, concentrate and access my memory.
The theory goes that an anxiety state will stimulate the early reptilian brain (limbic system), and the limbic system will then shut down the executive seat. The limbic system is far faster at processing stimuli which apparently comes in handy in the case were you find yourself being attacked by a lion. You need to react immediately - not have to think about your options of fight or flight. The same system used to save you from an imminent attack by a deadly creature can also activate itself in every day life, even one devoid of such immediate dangers.
My problem with mental function and even DP disappeared immediately once I'd understood the relationship between the limbic system, anxiety states and the way it was affecting my brain function. It seems that worrying about the DP/brain fog/etc are enough to keep the mind in a permanent state of over activation, and hence a perpetual state of DP/Anxiousness.
Paradoxically, the only way I overcame DP was to stop worrying about it.