I tried prescription memantine, both the generic and XR formulations. The brain fog startup side effect never went away for me, so I had to quit. I've never found a supplement that has actually done anything.
Do you have any nutrient deficiencies? Ever looked into nutrient tests like NutrEval? A comprehensive hormone panel? Thyroid panel? Genetic testing?
It helped in some other areas, but not being able to do simple tasks nixed it for me.