when to let go?

I'll admit, I'm not the most experienced person when it comes to relationships. I dated a lot in college, but never anyone serious, or longer than 3 months. I backed out, or never took it seriously from the beginning. Then I met David. We were set up through a mutual friend, so I trusted him more than the other guys I dated already. Our relationship was great in the beginning, but for the last six months or so it's been miserable.