Reiterating briefly some other good points, but, especially, first: probably a reasonable person's criterion for understanding anything (e.g., their own work) is that it seems so reasonable and inevitable that everyone else already knows it, and it is trivial to understand. I do claim that this is a side-effect of pretty-good assimilation of an idea... otherwise it cannot be sublimated adequately to be used efficiently.
The counterpoint to that is that, yeah, maybe one's ideas are just rediscoveries of old, old things, or are indeed already obvious to experts. Hard to know. All the worse when one oneself gets to the point of feeling that things are obvious... because that may or may not be a signal that they are obvious.
My meta-advice would be to try to not think in those terms. That is, if at every moment one is worrying about the novelty or non-triviality or... of one's reflections, one is squandering mental cycles in an essentially hopeless endeavor. That is, to my mind, the only sane enterprise (e.g., for an academic) is to try to further one's own understanding. (This is not a "problem-solving" approach, no, ...) Obviously, one's own understanding tends to lag behind collective understanding, whose advancement is one of the reasons we get paid. But/and, even with computer assists (currently), we cannot pretend to operate with "collective" understanding, but only our own personal understanding.
Yet/and now-and-then while innocently trying to understand what other people have said or written, one accidentally understands a thing which was not well understood before. That is "research", I guess. I really do think that that is the most sensible way to perceive our enterprise...