I have a decently large hand and my thumb is double-jointed. I find any way I position my fretting hand never feels right because of how my thumb bends. I’m trying to find a way to grip the fretboard without excessive pressure to play notes cleanly. My thumb and the muscles around it feel really sore after playing for a long time.
Does anyone have any advice?
Pretend you’re trying to eat a big mac with one hand.
You can practice playing scales slowly, one finger per fret using just your fingers and no thumb, then add your thumb in, behind the neck from your index/middle fingers.
I don’t have big hands and I’m not one of the lucky 33% who have independent ring/pinky finger control 
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Thank you for the response! That does help a bit but my thumb ends up bending back and it gets sore.
This is my natural thumb position in the picture. It helps on the right hand at least haha.
I’ve never thought about moving my fingers independently! How do I know if I can? I can move them independently a little bit?
That looks like Hitchhikers Thumb aka distal joint hyperextensibility 
which is a genetic, recessive trait.
Hold your hand open with your fingers out straight and see if you can independently curl your ring and pinky fingers to touch your palm.
If the traditional technique doesn’t work for you body and is causing pain, you might need to re-think the technique approach.
One way that I play to conserve my fretting hand energies is… with very unorthodox and frowned upon technique.
I grab the neck like a baseball bat. My palm is right up on the neck.
This severely limits mobility, but it provides support for the fretting fingers without relying on the thumb.
It makes using the One Finger Per Fret fretting technique impossible (where you use index, middle, ring and pinky individually) and only permits playing with the Simandl technique (where you use index, middle, and then ring and pinky together, covering only a 3 fret range).
It requires a light grip and lots of arm movement and hand shifting, but it may allow you to play pain free.
I wouldn’t recommend this technique to anyone unless they are experiencing severe limitations and pains when trying for the more traditional and much more flexible technique demonstrated in the course.
But I’ve had plenty of students over the years whose bodies don’t allow them to apply traditional technique with comfort or success.
And if I’m tired and stuck on a 3-fret-range groove for a good long while, I just might bust this out and relax my thumb and hand for a spell.
Here are photos of me playing on the A string, 3rd, 4th and 5th frets respectively.
The hand position changes and has to rotate to get to other strings, but this is the basic idea.
Lemme know if this helps at all.
Hi!
Sounds like you’re already experimenting, so you’ll find a way. I have the same thumb arrangement, and do what Gio posted when my left hand becomes too tense; and standard technique for everything else.
One thing about tension is that a short period of complete relaxation can buy you tons of time. So whenever you have a break, let go of the fretting position!
For finger exercises, this (very old) video is stellar. You can skip the crazy 80s intro, but this video is a must watch for finger flexibility!
I’d definitely try what @Gio recommended. I’m still trying to picture your natural double-jointed thumb at rest. I work with a few in the kitchen but that’s usually comes with advantage not the other way around.
When I work on my finger pressure exercises, I do it without thumb support at the back of the neck. I’d work on scales and up the tempo, when I place my thumb back on for support I notice the significant improvement on overall finger pressure. In addition, I find myself using a lot of thumb finger tip playing because it allows for more comfortable play on the D and G strings.
At the end of the day, I think you just need more time. How long have you been playing?
I’ve been working on speed exercises lately and i realized that i’ve developed a tendency (due to laziness) of letting my thumb point along the neck towards the headstock. That’s fine when playing around the 1st 5 frets but it causes you to rotate your hand so when I get further up the neck, it causes a problem using my ring finger and with moving efficiently between strings. I’ve been doing a lot of slow work consciously keeping my thumb behind the neck, across from my ring/middle fingers and keeping my fingers parallel to the frets so i can get a better spread on my fingers.
