I have a textbook on logic, I found it at the dump probably 3 years ago, during my second year of high school. I tried working through it many times, but simply could not wrap my head around some of the concepts it presents. In particular, I found its treatment of formal languages to be very confusing. I could, for the most part, follow the material as I read it, but attempting any of the exercises quickly became an exercise in futility.
Currently, I am in the so-called "Foundations" course (MA211) at my university. Currently it is covering basic logic: quantifiers, proofs, etc. Suddenly, I find myself able to tackle some of the problems from the earlier textbook. Indeed, it is actually starting to make sense. I find this feeling, the wave of understanding of a previously impenetrable subject, very pleasant. It usually corresponds with a massive increase in breadth of understanding, as I make connections to other subjects. Hopefully I'll get these moments more often as I intensify my studies.