From the marketing side, we call that finding product-market fit. (If you want to search for resources.) I think the CTO at the startup I work for is pretty good at it. But, I don't know how he does it.
Have you considered working with a tutor? Have you had any evaluations to determine if ADHD, processing speed (the human kind) or other issues play a role? A tutor would be able to tell you if your expectations are too high. An evaluation would be able to tell you if you need to adjust your learning methods.
Wrote this as part of a recent article, "The World Economic Forum predicts that by 2022, 75 million roles may be eliminated due to technological changes, including automation. However, other advances may result in the development of 133 million new roles requiring new skills."
Over 11k words, this piece begins with an admonition to always keep growing and gaining new skills as a professional followed by explanations of cognitive theories and learning strategies that will make self-learning easier and more efficient.