Here's my theory about "bereavement brain" (devised since losing my husband nearly 2 years ars ago): when you're a couple, you kind of share the mental tasks. You borrow the other person's brain to find things out ("When is the car MOT due?") or you park stuff there ("Right, I'm putting the suitcase keys in this zipped pocket of my bag, OK?") or you use the other person to set a reminder ("Don't let me forget to ring the surgery when we get home!")
Then when we find we're alone, possibly for the first time in decades, it's no wonder our brain can't cope with it all. As well as having to filter out the painful things in order to function normally, our brain now has no spare capacity to draw on, no backup system. That's one reason, I think, why we all sometimes find ourselves floundering more than usual. Oh, and we're steadily getting older all the time too, which doesn't help!