Did you try thebookofshadders? I thought they did a pretty good job getting you through the basics of GLSL. (although the journey does not end there. :))
Yes I went through it, it's an amazingly great resource ! But yeh the journey does not end there... I should just focus and set myself on practicing it for a while.
Doing the same here. I went back to my old algebra classes, which I feel helps. Their are some good ressources on youtube, too. Check out 3blue1brown's linear algebra series for instance.
