A foggy mind that struggles to focus often isn't a sleep or a stress problem. Your body carries a load of everyday wear that builds up quietly over the years, and a clear, sharp head is one of the first things to feel it. More sleep doesn't fix it, because the fog was never really coming from how rested you are.

You can try your hardest to focus and still feel your thinking slow down, because the fog comes from that build-up inside, not from a lack of effort. That's why pushing harder never clears it, and why it can show up even on the days you slept fine and had nothing much on your mind.

The good news is none of that is permanent. Clear the wear that's been clouding things, and a sharp, steady head tends to come back on its own. There's a simple, natural way to go after exactly that, and most people that try it start to notice its effects within a couple of weeks, thinking clearly through the afternoon instead of fading by the middle of the day.
Based on everything you've told us, here's what we'd point you to.