Co-dependency. Once you get together with the devil you know, you might find yourself hooked on his or her drama. The dynamics of co-dependency are complex, but while the tremendous costs of your relationship are obvious to everyone, you may also be getting a significant hidden psychological "kick" out of the relationship. This can affect your brain just like an addictive substance does!
Low self-esteem. Low self-esteem is another huge factor in staying together with a bad match. Many people prefer staying with someone who is not good for them to being alone. Dealing with a messy relationship might be easier than dealing with the anxiety and depression that comes from loneliness.
Abuse. Abusive partners have a way of getting into your head. They have their own version of reality, and they blame you for everything. As a consequence, you might find yourself going back and forth between believing that it's all your fault, and realizing it isn't. Continuous put-downs can suck the energy out of you, leaving you too exhausted to do anything about it.
So, where do you go from here? First, don't assume that things will get better on their own. Second, stop trying to deal with it alone! Surround yourself with people who are supportive. Third, if there aren't a lot of people you can confide in, don't hesitate to consult with a professional to hash out these problems. Whatever the reason for your bad relationship, it's okay to get help, both to end it, and to have better ones in the future.