I read an article once on the art of finishing. It was more how to end something than finish it. If you are several chapters into a book and you are not enjoying it, put it down and read something else. Don’t force yourself to keep going in the hope that it gets better. If you are 15-20 minutes into a TV show or movie and you are bored just turn it off and go find something more interesting to watch.
There are so many books in this world that there really is no excuse to waste your time reading something boring. There are only so many books that you can read.
Take the average life expectancy for your country and gender, minus your current age, and times by twelve. Now times that by the amount of books that you generally read in a month. That’s how many books you have left to read, in theory. I’m guessing that it’s not a huge number. So why waste time on books that don’t interest you? Fill the time with books you enjoy. There are only so many books you can read.