I never feel guilty about not using them and I seldom sell unless I find myself with too many of one thing, need room for something different and new to me, or it is clear that someone wants them more than I do. Once they've been cleaned and oiled they don't require much upkeep, so keeping them even if I don't use them or particularly want them anymore doesn't seem like a wrong answer.
There are certainly firearms that I regret selling, but what's done is done.
Hindsight is always 20/20, foresight not much.
