The challenge was that the textbooks and material that is popular among University CS courses leave a lot of unanswered questions that I couldn't get over. For example, when I came to hash tables while studying data structures, I couldn't figure out how the hashing functions did their magic. Every book or online lecture I came across showed how to use a generic hashing function and moved on to dealing with hash conflicts. I couldn't move on because I kept coming back to questions like "ok, but why this hashing function? If I change my data a little bit, is this still the right hashing function to use? How can I tell?"
Somehow, people who had formal education managed to sail past these questions. I suspect that having someone to answer these questions when they came up played a crucial role in them understanding the material.
My point is, some of these materials are almost deliberately designed to be consumed with an instructor who can answer questions. Especially for the difficult courses.
In the end, when I was teaching myself the basics, I had to go hardcore and pick up TAOCP to keep up. TAOCP has a reputation for being a very dense reference book and being unsuitable for self study - but I found the opposite to be true. Prof. Knuth painstakingly covers every minor detail leaving nothing unaddressed. (His treatment on hash tables has several pages on hashing functions that gave me a clear understanding of how things work). I would recommend following that path if you are trying to study the basics without formal instruction. It's a little harder, but you are more likely to end up successfully digesting the fundamentals and move on to more advanced topics.
Today, I have fully rod myself of insecurities, and I have developed a habit of going to the most comprehensive treatment of a topic if I'm trying to teach myself something difficult. For easy subjects, textbooks popular in Universities is usually good enough. For difficult stuff, I prefer going hardcore.
Be aware of textbooks designed to work with formal instruction.