I used to be, too. Here are the two easy steps I took to break that habit:

1. Start reading Atlas Shrugged. When John Galt starts making his speech, skip to the next chapter. You have now gained the ability to skip pieces of a book that are really, incredibly tedious.

2. Start reading Twilight. When it starts feeling like torture to continue, stop. Use the rest as fire starters for your grill. You have gained the ability to not read books you discover that you don't like.

Take those unfinished off the shelf and get rid of them. Sell them to the used independent. Donate them. Burn them. Landfill them. Whatever. You can reclaim that shelf space for books you not only want to finish, but re-read and finish again.