Swiss cheese holes in my education. Do I go back and fill them?
First, for context, I'd disagree strongly with a too strict notion that "mastery" is required before "moving forward", for several reasons. It is easier to understand the purpose of something after one sees how it is used. It is all too easy to acquire a fake mastery that is not functional, but only refers to some artificial tasks created for a textbook, and, all the worse, may be wastes of time. And, finally, the kind of "mastery" we are often led to think we should attain is basically unattainable... That is, unless one keeps ones list of things-to-master really short, very elementary, and uncomplicated, this "mastery" is just unreachable on the terms a novice or even journeyman conceives.

So... don't over-interpret "mastery".

On the other hand, _of_course_ it is good to go back and fill in. And one will find oneself doing this many times, as one discovers, upon "stressing" one's knowledge, that what one thought one knew "well enough" was not quite good enough. 

For that matter, even with an excellent memory, things slip out of a person's head.

My bottom line recommendation is to maintain (at least) two "threads", one to review and backfill, the other to move forward.

The latter is surprisingly useful at helping review make more sense! :)