As a teenager I had an unwritten rule* about never giving up on a book I'd started. I suspect it was an inner wish to not feel like I'd wasted having read half a book. Some silly pride thing, no doubt.
Well, if there's one thing I can thank Stephen King for, it's that he cured me.
I think I struggled to about halfway through The Tommyknockers when I had some very thorough and shallow inner searching, and very quickly came to realize that the time had come.
I put down the book.
Went to sleep.
Got a new book in the morning.
Still, it did take a while longer until I really felt comfortable with such decisions, and I'll agree that sometimes it's worth struggling through, as some novels will pick up, or you'll fall into the right slot to activate the mind in a way that makes it interesting.
A bonus to getting more comfortable with this is that it's easier for me to put down a book that I actually like, to go back to it at a later date, because I feel I'm not in the right mood at the moment. People speak of enjoying different things at different seasons, so I suspect it might have something to do with that, or just the current mental state in general.
It's not like putting down a book because you don't like it is a big deal. If nothing else, you might always go back to give it another chance a few years down the road, to see if it sits better then. But I'm done with forcing myself through tripe.
*While I was a geek, I thankfully didn't go as far as writing down such rules. Phew!