A 58 Old Git here. I tend to get pain if the neck is too shallow, or there is too much relief / concave bend in the neck.
If you know this stuff already and I'm stating the obvious, sorry about that.
If not, does it get worse in the Summer ? Humidity is higher in the Summer meaning for the same amount of string tension / pull , the neck will bow more as humidity makes wood more flexible.
I use a degreased spark plug gap gauge to measure neck relief / bow. Put a capo on the first fret. If right handed, put your left hand forefinger on the 17th fret.
Measure the gap between the string on the 7th or 8th fret with the feeler gauge with your right hand., depending on which is bigger.
The bigger the gap, the harder it is to press the strings down, hence the fatigue, the difference can feel severe.
Often new guitars come with too much relief. Both humidity and trying to hide uneven frets can be the cause.
I lived in a really humid house which is why I found this stuff out.
It depends on guitar model what the recommended relief should be. The more level the frets. The less bow you will need. Too little you can lose tone though.
Fender recommends for a 25.5 inch scale, 9.5 inch Radius. 0.010 inches, I tend to try and get 0.008 if I can. They recommend 0.12 for the vintage 7.25 radius.
Gibson recommend between 0.008 and 0.012 my 24.75 Scale length guitars, some of mine have less relief. It just depends how good the fret job is.
The cure for too much relief, turn your truss rod if looking at the headstock end down the neck. Turn it to the right, this will tighten / straighten it to compensate for the extra bend in the neck.
Go the opposite way if the neck is too straight., it doesn't sound like the case here though. This loosens the rod. Just remember, Righty Tighty , Lefty Loosey.
Do a little bit at a time and rest the guitar as the wood can continue to move. All truss rods in the same model seem to react differently as all wood is very different.
Where the wood is cut from on the width of the trunk of the tree makes the flexibility vary wildly and how the tree is sawn makes it differ too.
It is more of a task on a vintage style Fender neck that alters at the body end. You really have to take it apart and when you put it back together, the string tension will be looser and have to resettle.
Why I just don't do vintage style Fender necks anymore.
They are a P.I.T.A.
I'm hoping this is the answer to your painful problem.