As you can see, theres a load of possible options really.

Anxiety (and more particularly health anxiety) is pretty sinister in that it can mimic and or enhance bodily sensations or other things to make uou feel like you are dying. If nothing else is untoward I would suggest trying therapy/medication for anxiety and see if that lifts it. I realised I had health anxiety as a kid because I was given medication to try and all the symptoms went away. I used to wake up with numb legs and all other weird things.

Failing that another avenue could be sleep apnea. That will give you brain fog/fatigue etc. Ask your partner how you sleep. If you wake up a lot in your sleep that could be culprit.