To finish or not to finish?

I am reading a brick-sized book at the moment and I think I'm almost defeated. I used to have a rule that I had to finish any book I started, but when I told my mum about it she pointed out that there are so many great books out there, why spend time reading not-great ones. Now I'm getting bogged down in the middle of an enormous tome. I enjoy most of the book, but there are a litany of reasons I'm starting to give myself not to continue: I've renewed it twice already, I don't really care about any of the characters, I don't feel like I'm learning anything (personal or about writing) from the book, it's so big that I can't read it in bed, I've read another quick book just for a break from the drama of this book and I have a book club book I'm supposed to read. But then this little, stubborn voice says, 'You don't want to give up on it, do you?' In principle I like finishing things I start, even if they have no greater significance than my own principles. In this case that's kind of annoying.