Over-taking notes due to: 1) copying sentences in textbook + 2) trying to fit new understandings in previous notes which just makes things messy
Let me make a few suggestions:
First you want to understand how the mind works in order to know how to optimize your study. It turns out that to learn, we actually need to physically change the brain, reconnecting neurons. For the science behind it see: The Art of Changing the Brain by James E Zull. The implication is that we learn by reinforcement (repetition) and feedback. The feedback is to assure that what we have &quot;learned&quot; isn't actually wrong.
Having a complete set of notes on paper (or in your computer) doesn't imply that you have actually learned anything. You have to make the connections in your brain so that you can both recall things and put them to use.
Reinforcement is actually increased and enhanced if you take notes in your own words, rather than copying them. The original words can give you feedback if you are studying on your own, but using your own words implies that you were more engaged when you were note taking and not just copying without thought.
So, paraphrase and then check.
The second bit of advice is to periodically go over your notes and summarize them in a smaller space. What do we now see, having read a bit more, as the most important ideas in our notes? You can take this to several levels.
Another trick is to use index cards to effectively capture the essence of an idea in a very small space. See this answer to a different question for an explanation of something called the Hipster PDA, which is both an effective tool for note taking and for summarizing. Don't write too much on a card, though. The idea is to filter things down to the essence.
And cards are especially efficient as they can be rearranged (to combine ideas) and also easily carried to help you with the reinforcement. You can carry a dozen cards and review them while waiting in line, for example.
There is nothing inherently wrong with long form copying of sources for your own use, but there are more efficient ways to assure that you actually learn from your reading and not just move it from one bit of paper to another.
Reinforcement and feedback.