In engineering school, I had to learn mechanical design and 3D modeling with Catia V5R20. I had 35h of classes on it with real-life teachers, then I worked on the solar car of the school and did some modeling there. I became quite good with it.
Before getting out of school, I tried to learn the competitor, Autodesk Inventor, which is much more elegantly designed and has a yearly license fee much more realistic for a freelancer. Also, they give students free licenses for 3 years. Autodesk has awesome doc, with text and video, most of them translated into my native language (not English…). But at that point, I had mostly specialized in coding thermodynamics software and lost some motivation and interest for mechanical parts design. However awesome the doc, there is still a lot to cover to get a sense of the internal logic and I never reached a similar level of mastership in Inventor than what I did in Catia. Also, not having a teacher makes learning incredibly slow.
I have used KDEnlive as a video editor for years now. It sucks at color science, it used to be super unstable (much better now), and used to be super slow (minimal GPU makes it usable now, or makes the output completly garbled, depends on days). Then I tried to learn Da Vinci Resolve. Again, awesome docs, PDF, website, videos… Then I tried to edit my 10 FPS screencasts in it, they are not supported. I have some more serious artistic-ish stuff I need to edit, I’m still procrastinating doing it in Resolve, and still using semi-shitty KDEnlive when I must.
So. Learning.
First, you need a real motivation, as in : a goal.
Then, you need some real dedication, as in… a really important goal.
Then, you need some really real dedication, as in : the courage of not falling back to the semi-shitty stuff you already master that doesn’t work well but gets you some results now.
Finally, you might need someone to answer your questions in real time and guide you through shortcuts and personalized pieces of advice because docs are a reference material, not a class, not a handbook, not a lesson.
To use a doc properly, you need some prior idea of what you try to achieve and why. Which brings us to something very unoriginal : school, teachers, seminars, internships and the like.