It's very easy to lock yourself into using too much force when playing guitar, especially when you struggle with something. While in reality you wanna be as relaxed while playing as possible. For so many years I did not realize this simple thing and it had an utmost detrimental effect on my playing, demotivated me, and made me abandon guitar for significant periods.
I can't recommend this video enough.
It helped me to realize how paramount relaxation of the whole body actually is. Realizing this helped me further to analyze and fix numerous problems with my left hand technique.
Barres were especially painful in the past.
There are many materials on this subject and quite many talk about the same things: using natural weight of your hand, not pushing with you thumb stronger than you're pulling with the rest of your fingers, but I still could't figure out how exactly it is support to work until I watched this video and it finally clicked with me.
Here Jack Sanders gives great examples and explanation of correct and incorrent techniques which helped me a lot to work through the issues I was struggling with.
There is also another video from another person about a really nice righ hand technique, which I will post later in another thread.