As well as the physical side, there’s also the mental aspects of learning the guitar.
The best lesson I could of had would have come before I got my first guitar and needed simply to say “You are not James Hetfield”.
I remember reading an interview where I read he only downpicked. And so did I, to learn a few Metallica riffs. But the problem is I wasn’t James Hetfield and so I subsequently only used this technique for all future playing.
I had fallen into the trap of thinking I could sound like who I listened to. If you practice a lot (and better still have talent) you may play their songs like them but not sound like them. That’s the point most beginners miss. You need to find your own style and sound. That usually takes a long time so copying a particular guitarist is the usual route in. Unfortunately most guitarists don’t progress beyond this stage and just sound somewhere between a poor imitation and mediocre copy.
After a couple of years learning by myself I could play a few riffs (but not a whole song) and thought that was I was doing ok. Until I had my first proper guitar lesson. I sat there and my teacher said ‘this is in G, play along’ and I realised that I actually had learned virtually nothing, apart from a lot of bad habits which took ages to undo.