Looks like the other comments covered have made quite a few suggestions. (This is the only post of yours that I have read, by the way.)
My thinking was that a mild brain injury (which could produce the symptoms you speak about) does not usually show up on an ordinary brain scan. A functional MRI is what you want (and there might even be something that supersedes a functional MRI, to show a mild brain injury).
You do not need to have been knocked unconscious in order to produce a brain injury, nor have been in a coma. Whiplash from a car crash - or even just banging your head into an open cupboard door is enough. Which you would not likely remember three years after the fact.

You might want to check out a brain injury association in your area to see if they have any suggestions about the cause. Better than that, I know they would be able to teach you some coping skills to help with your symptoms. Lash Publications has a wide variety of publications on brain injury - some of which are free - that might be helpful.

I hope you get some help from a comment here.

Yay for you David! And thank you so much for your posts this year. Your bravery will help many face this in their lives.

Life is a bumpy uncertain road! I'm very happy for your news. It is good in my opinion. Thank you for sharing your self and thoughts with us.

Thanks for sharing. Are you a Veteran? PTSD has similar symptoms, not easy to diagnose... my husband was diagnosed with Alzheimer's in 2008, had symptoms for a few years prior to diagnosis, a year after AD diagnosis he was also diagnosed with PTSD, a result of his experiences in Viet Nam....many traumatic events can also trigger PTSD...hope you get definite answers, maybe this or maybe that, must be frustrating for you....

Uncertainty sucks!

It sounds like you are continuing to make lemonade when life hands you lemons. Keep writing.

Some of us docs don't know that certain meds - even thyroid meds - that have been stable for years may become overdoses as their metabolism slows down with age. Something they have tolerated may no longer work well, e.g. Benadryl. Lower doses make sense, and I would not go to any doctor who acts like they don't believe in side effects. I suppose having had to take some meds myself and learning about side effects first hand was a blessing in disguise...

You are so right Onlydotter..I have a very ill relative who lives 1,000 miles away and I have told her idiot son to look into whether the statins she is on and really any of the meds she is on, and it's a lot, but I have told him to ask the drs to test her to see if they are adding to her confusion and mental decline, but it's like talking to a wall with him; he's one of those arrogant thinks he knows it all kind of boobs.

Also people do not realize there can be toxic build ups of meds they are on and just because a person has done fine on a certain med or a variety of meds for a while does not mean they cannot cause bad side effects eventually. Sometimes side effects take a while to happen.

We, unfortunately, live in a very over-medicated society; plus, who knows what new side effects meds can be causing, especially if a person on them is already confused and/or elderly, often the "brilliant" drs decide it is from aging not the meds. hey blame it on getting older, when it could very well be a bad reaction to medications. A lot of medications are toxic anyhow. Ugh to this world where drs think they are gods and no one can tell them anything, nor give them advice. There are many drs who think they know everything and pooh pooh us "lowly" humans who very well might be right in our beliefs over what is going on with our loved ones concerning meds, interactions and toxic build ups and adverse reactions to the meds. Sometimes family members see and know things drs do not!

vstefans wrote "meds ... can make you have odd lapses and affect working memory and maybe motor planning, while not really making you stupid (e.g. topiramate, statins - I had a rather bizarre but fortunately fully reversible cognitive experience with the latter...") statins DO have a reputation and even a warning for contributing to mental decline, according to WebMD: "Statins also carry warnings that memory loss, mental confusion, high blood sugar, and type 2 diabetes are possible side effects...may also interact with other medications you take." Most statin side effects are generally considered reversible, although they can be pretty terrifying when they happen. My father had a muscle reaction, probably from elevated CPK, basically paralyzed his legs, and after a week in the hospital, an extended recovery period at home. -- April

Wow. My first thought was maybe just you are very bright with so much "cognitive reserve" that you tested within normal limits. Depression can do this, too...I actually had myself neuropsych tested at one point, after having some really scary jamais vu expereinces, getting lost in somewhat familiar buildings when minor to no changes had really been made, just to find out that I wasn't losing IQ points or my mind - I was just stressed, depressed, and left-brained as hell to start with. Are you on any meds that can make you have odd lapses and affect working memory and maybe motor planning, while not really making you stupid (e.g. topiramate, statins - I had a rather bizarre but fortunately fully reversible cognitive experience with the latter...) Unusual diet? Unusual amount of caffeine? Migraines or subtle seizures could do it too. I bet there's an explanation, and it could very well be a much better, more treatable thing than early Alzheimer's.

I mainly want to send you a hug. You are going through a tough time with your good news/bad news.

Are they considering types of dementia that would not necessarily show up in a brain scan? As you probably know, Lewy Body Dementia is often called roller coaster dementia, because the cognitive and behavioral symptoms go up and down without warning. Test results can look better or worse than the patient's overall day-to-day symptoms, depending on whether the test is taken on an "up" day or a "down" day. I sure am not qualified to suggest a diagnosis for you! But I am curious as to whether LBD has been specifically ruled out, and if so, on what basis. I do a little volunteering with caregivers of persons with LBD, and I'm always interested in learning more about how it is diagnosed/ruled out.

Could your cognitive decline be from depression, or unresolved grief? Do you feel unfulfilled in your life , if so, has that made you kind of give up and your brain/mind has reacted to your thoughts by not functioning as well as it should? Do you have what you consider a stressful life? Are you in an unfulfilling relationship? Do you like your job? Or if you are retired, do you feel like you would like to be more active? Could you have a weird food allergy? Could you perhaps have Lyme Disease, I know that can cause some mental impairments. Do you regularly check your blood sugars? If not perhaps your blood sugars are off. Could you have a UTI? Could you have an infection somewhere in your body or brain? Do you get regular dental check ups? If not could it be from bacteria in your mouth? I have heard of people getting brain infections from a badly infected tooth. Did you smoke pot a lot as a teen and young adult? Do you drink alcohol or did you ever have a drinking problem? Those behaviors can definitely do things to the brain. Have you had the oxygen levels in your blood checked? Have you had your cortisol levels checked?

I have only read this one article you wrote here, so if you did look into all I wrote, sorry, I did not read any of your other writings. You could also Google "Reasons for cognitive impairments" and "Reasons other than dementia for cognitive decline" and see if anything written could apply to you.

Thank you very much for sharing, good information. My mother and dad are mid-eighties, both diagnosed with cognitive impairment, Mother's probably vascular dementia, and Dad's consistent w/ very mild Parkinson's, keep me on my toes as regards dementia and Alzheimer's developments / information. I am a nurse, and I do nutrition research and counseling. I found my own memory improving with lower glycemic diet in the form of smoothies and "soups" with several raw produce ingredients, a quality whey protein, and healthier fats such as coconut oil, flax, chia, and olive oil. All the above foods help keep blood sugar low-normal, avoiding blood sugar rises and spikes, consistent with recent findings associating greater dementia risk with high-normal (but non-diabetic) raised levels of blood glucose. -- A.J.

Two thoughts come to mind......... possibly a thyroid problem or a sleep disturbance such as apnea causing your symptoms. Good luck to you!

Thank you for sharing this most recent news, I am looking forward to your future posts. Please know that your willingness to share this journey is encouraging to me in many ways.