I find this as well. My dad is an English teacher so it was drummed in from birth haha :)
I find using a computer so much doesn't help as I type fast and rely on auto spell check to get things out faster without having to think so much.
I find I always tend to type certain letters in certain words the wrong way round, unfortunately solicitor is one of them which is a shame as that is what I am! Bit ironic really!
Past and passed is one which has bizarrely started to confuse me recently! Daft as it never has before.
My brother and his wife do a lot of crosswords and sudoku and such as they read that keeping your brain active helps stave off things like Alzheimers and dementia.
I have to admit, I don't read nearly as much as I did, hardly ever read books for enjoyment anymore, don't do puzzles like crosswords like I used to and don't get a daily newspaper anymore as I watch Sky News instead or read it online.
Saying that I read a lot with my job.
Maybe the more you knew in the first place, the more you have to forget :) I guess our brains only have so much space!