Interesting question. It was hard but I finally learned to let go since there are just too many good books out there to waste time on the bad (just because of series considerations

).
Gave up on Star wars (one large series if you think about it) after the Legacy of the Force debacle. Ditto Eric Flint's Ring of Fire series when it starting taking history far too seriously (NTTAWWT

). Dune, after (well, during) God Emperor of Dune.
It's the "waiting for a bus syndrome" sometimes. Well, I've waited 10 minutes, seems a shame to give up now. Sometimes you just gotta take a cab home

.