I find the same problem, also I'm around your age so I can relate. Sometimes I feel like mathematical papers are written to be complicated. However, I think that most of the problems concerning being able to reproduce an argument given in a book or paper has to do with not understanding the argument in English. What I mean, is that people (me especially) have a bad habit of understanding the symbols but not being able to explain the concept to a 5th grader. For instance, if you read about the proof for the fundamental theorem(s) of Calculus, you might just miss the entire point of the argument in trying to understand the symbols and notations of the argument. In reality, understanding the argument has more to do with being able to translate mathematical jargon to English. And if you want to be able to write proofs, you have to be able to translate the English into mathematical language. This is easier said than done, and I'm sure most never really learn how to do it, that's why we have so many horrible mathematical papers. But if you learn how to do that, you won't have to memorize math, you'll only have to memorize concepts.
Also, don't think that these mathematicians that use complicated jargon are smarter than you, they most likely are similar to your intelligence. They have more experience, but in the end, they'll mess up in ways very similar to us. So if you apply all the above and still don't understand the concept or argument, move on to a different source.