In general, this is psychological (emotional, mental , whatever you want to label it).
It has a very low chance of being neurological in nature.
Why?,.. I just read what you wrote. Your question and description is written better than 90% of the responses/questions on Y!A -- you have not lost your intelligence, and can still display it quite well.
I know how you feel,.. and I even know that you probably are displaying a lower level of functioning than you used to, say, a year or two ago. You feel it, but what others see is NOT directly proportionate to what you feel.
Generally , I have discovered overly analytical personalities, and people with relatively higher than average Intelligent Quotients tend to manifest their depression and anxiety problems through a neurological mime. In otherwords you experience what is somewhat a mix of depersonalization combined with your brain being occupied consciously and subconsciously with a barrage of analyzing and re-analyzing your situation in life, what you just said, did, didn't do, wrote, tried to think of, forgot, what did you eat for lunch, did you even recall driving home, why don't you care about anything anymore, what if (insert the million health problems possible), it is miserable being dumber than you used to be AND why are you aware of it -- aren't you supposed to just be dumber and because of such you don't realize it.. other people do??, why am I stuttering, why is what used to be things I didn't even think about .. now are challenging?, is my vision getting worse (proceed to test it), why do I ache more?, this conversation is boring me, I feel like I am just going through the motions in life now, whats the point?, where did all this come from?, why me?, is this going to get worse?, etc -- you get the point.
Basically your depression/anxiety/ possible OCD (can be obsessive only.. you don't have to have compulsions).. and any other mental problem that can crop up.. is literally taking up a great deal of your CPU, processing power.
If you have 100% to utilize, your mind is using 80% or more just thinking about everything, analyzing, re-thinking, worrying, being confused etc (even if you aren't actively thinking of anything consciously).
As a result I bet you feel unfocused all the time.. like you are zoned out,.. almost a day dreaming feel but you aren't really daydreaming.
Do you find it hard to read ? Literally -- from literally keeping focus on the words to remembering the last 3 words you just read? Having to re-read?
Is maintaining focus in general something you have to FOCUS to do?
Do you question 90% of your actions.. analyzing them to see if you screwed up somehow,.. or others thoughts?
But, do you still make decent grades? (I assume you are in school) -- I bet you do.
If you aren't in college, well.. do you still manage to drive from A to B ? (Operating a car takes an incredible amount of brain power) -- Do you still manage to be able to do your job? You can obviously still operate a computer, navigate the web, and write exceedingly well.
What I am driving at, is while you are only running at , at best, half capacity/power -- You still have the potential and capacity to do these things. Which exemplifies your overall intelligence.
At any rate -- bottom line is this:
See your GP/PCP -- on first appointment discuss ONLY the neurological impairment, how it is interfering with your life, the seeming 'dementia like symptoms' -- Try your best not to clue in on a depressive disorder or anxiety issue. Give issues that point directly to an inability in the brain. IE: Something like an ischemic (low blood flow) condition to the brain, or other 'brain' factors.
Your goal is to walk away with no medication, but with tests. At least comprehensive blood work, and ask for a referral to his favorite/most respected neurologist. (In a way that isn't disrespecting his knowledge, but that it seems to be relegated to that field.. so where would he send you.)
If you get to the neurologist, explain the problems briefly, and he will do basic neurological exam in which you will pass just fine. Inquire further about MRI/imaging to rule out any underlying issue.
If you can somehow come away with the blood work (and any other testing) from your PCP, and a good neurological workup and an MRI with contrast from a neurologist -- and everything comes back 100% normal (which is most likely will).
THEN let the psychological/depression (whatever is ailing you) out of the bag to the GP/PCP.
At any rate, you can be "cured" in time -- just start on it now.
WITH the medical route, I also recommend the following:
1) Exercise -- both resistance and cardiovascular.. several times a week at least.
2) Get out of the house/apartment, etc every single day. Don't stay in one place all day.. basically don't isolate at all.. it will only work against you here.
If you do nothing else right away -- at least get some cardiovascular exercise 3 times a week minimum and do not isolate at all. Those are crucial.
I know your misery,.. it is something that you can describe to others and they just can't 'get it'. They might say sorry, or something else empathetic -- but they have no idea unless they'e been through it. Even your parents,.. if it goes on long enough will begin to tire of you having this 'thing' that they can't see (you look fine) and isn't diagnosed as anything.
Just remain hopeful because it can and does get better -- but being proactive will make or break things.