How can you lose such simple things from your mind?

After all these years as an almost unbeatable speller, as someone who just KNOWS how words are spelled without having to struggle or recite or lean on mnemonics or other tricks, I have suddenly been beset by a crisis of confidence in my spelling of “therefore,” meaning “as the result of.” It has been tripping me up and leaving me in a cloud of doubt for weeks. I woke up one day and sat down before the keyboard and just didn’t remember anymore how to whether it was “therefore” or “therefor,” and I have to think it through to choose the correct one. I almost never mean “for, or in return for that;” I mean “for that reason.” The last time I can remember being this troubled was by “guarantee” in the 3rd grade. So now I’ve got it back, but I have to think, “it’s NOT ‘therefor,'” which is an annoying middle step.
So aging means broken heels and lost spellings?