Brain overload is common in PPL training. I recall being like a zombie after a flight.
The way PPL training is done it is quite hard and not pleasant. If you want it easier and much more enjoyable, you take the instructor on trips and learn as you go along. Lots of well-funded people have done that. The problem is that you need a lot more than the 8k the PPL normally costs. The instructor will love you of course - they rarely get a chance to fly very far.
Carbon monoxide is a serious suggestion - well worth a check. A cr*p headset is another possibility - most headsets used in the training scene are cheap knackered types.