You know, a few suggestion for you since I had seen quite a few golfers went through what you are experiencing.
If you used to be a decent around chipping and putting, but experiencing changes now. This phenomenon could be attributed from a few things, definitely not your nerve as we were lead to believe.
No precise diagnosis, these are just a few possibilities which you can find out if they apply to your case.
-have a trust worthy optometrist exam your vision.
-have a physical therapist specialized in sport injury exam your hands and shoulders. Perhaps referring you to further exam from other specialist for injury resulting from repetitive activities.
Seek a trust worthy optometrist as you would seek someone to operate for open heart surgery. As we age, our vision will deteriorate over time from all the hours under the Sun on the golf course. What you see will lead to your decision on executing the golf swing. All great golfers have sharp vision. I seriously believe the famous celebrity golfer Jack Lemmon who was dedicating his life besides his work and family to golf had never achieved his dream of making the cut for the Bing Crosby Claim bake ( now AT&T Pebble Beach Pro-Am). He had poor vision.
All top athletes in their field of sports have sharp vision. Without exception. So your vision had probably changed since your 46. In a subtle way not enough to influence other activity just yet.
Have someone check your hands ( carpal tunnel ) , this will happen to most the golfers from hitting thousands if not hundreds of thousands of range balls. The vibration and shock wave will damage the nerve to give that involuntary twitch. I had my left hand done and it made a difference in how the hand feels comparing to the right without the correction.
The shock wave from hitting the golf ball and the ground will travel through your hands, arms, shoulders, and to your spinal core. Injured core will interrupt your motion through the complete golf swing. Especially those finest move like 1/2 to 1/4 swing. The chain of motion is broken down because subconsciously your brain will try to stop you from hurting yourself further. I was diagnose with such and had gone through therapy for about 8 months, still doing the exercises they taught me every week.
If you have a bad experience of not able to execute the golf shot like you know that you had done it a thousand times in the past with ease, that, will put a question mark in your mind. Like you said, like a cancer and it will grow on you, lurking in the back of your mind every time you’re in the situation.
I’d bet those might be the cause, and if you’re able to pin point the cause, the healing will take a little bit of time.