I'm in my late 40s. There was a time not too long ago where I felt the same as you. My mother died from early-onset Alzheimer's, so that spectre was not helping my anxiety about it.

Long story short, it turned out to be sleep apnea and some electrolyte issues. I now feel as sharp as ever. If you haven't seen a doctor about it, don't be afraid to. There's nothing wrong with getting it looked in to.