For years I finished every book I started, out of something like duty. Then I worked out how many books I would read in my life, and stopped wasting them.
The fifty-page rule
Give a book fifty pages. If you are not curious about what happens next by then, put it down. (Some readers subtract their age from a hundred and read that many pages instead. Same idea.)
It is not a failure
A book that is wrong for you now might be right in five years. I keep a short list of books I stopped, and sometimes go back.
What to do instead
Pick up something you are sure to enjoy. Momentum matters more than any single book.