A few years ago I helped out two other professional guitarists with similar problems and they too ended up rebuilding their technique and credit me with saving their ability to play. It is often the very enthusiasm and dedication to the instrument that gets musicians in trouble because they practice even though their body is trying to tell them something is wrong. I remember a video of Isaac Stern one time saying that every hour you practice incorrectly takes a couple of hours of practicing correctly to fix. Sad, but true.
I'm not sure exactly how I figured out guitar technique, but starting on the electric bass guitar might have had some influence. I was fifteen or sixteen when I finally got interested in music. I wanted to play the drums but my mother, after visiting the local music shop, came home and said "we can't afford to rent drums for you, they cost $10 a month!" Let me hasten to say that this was 1965 and my mother got paid about $250 a month. Our mortgage was $80 a month. Long time ago! You could rent a bass guitar for only $4 a month so that's what I started on. As my mother said "the bass guitar is also in the rhythm section!" She was a fiddler, by the way. So, apart from some abortive piano lessons when I was eleven, the bass guitar was my first instrument. Four very big heavy strings that you play with your right hand index and middle fingers. Pretty demanding for your left hand as well--you need lots of muscle.
Soon after I took up the six-string guitar as well and a few years later switched to classical guitar. I have a photo of myself taken in 1973 just before I went to Spain to study with Maestro José Tomás, so this is me before high quality instruction:
I'm playing a cheap student guitar, but the interesting thing is my hand position. For both hands it is quite good, no over-extension or obvious tension. There are a host of things that go into being a good player, but one of them is some basic physical aptitude, what my mother called "touch." She was a naturally good fiddle player and she used to say that so-and-so had a good "touch" on the instrument. That's a quality of string players and, I guess, keyboard players as well. I suspect it is something you really can't teach. Another element that is important is your sense of timbre. How aware are you of the exact kind of tone color you are producing? I have had students that were good in every other aspect, but they just could not seem to make a good sound. Another element is your sensitivity to the phrase. This is a purely musical element, of course. How you feel and direct the flow of a phrase comes directly from how you feel the music. On a higher level it is how you handle the structural flow of the piece.
With great musicians all the elements are present and reinforce one another. With pretty good musicians they have a lot of them and have to work on whatever ones they are weak on. So being able to analyze your own strengths and weaknesses is a crucial skill because you really can't count on anyone else, even a master teacher, to do it for you. And that's about all I have to say on the matter!
Here, by way of envoi, is Carora - vals venezolano by Antonio Lauro in my recording: