For me this question has a clear answer.

It is simply fear.

There are two main categories of fear that keep people in realtionships that are bad for them:

1) Fear of loss - You're going to lose the comforting history and measure of certainty you have developed.
2) Fear of lonliness - You're going to face having to find another relationship and you are worried you do not have what it takes.

These two fears conspire to keep you "hoping" things will work out and never facing directly the mounting and obvious truth of how bad things are.

If you become paralyzed with fear you have to use optimistic thinking to escape the trap. Pesimism can keep you in the bad cycle indefinately.

Examples of how pesimistic thinking let fear keep you trapped:

1. This relationshiped failed and its my fault - ill just ruin the next one to.
2. Im worthless and I should feel lucky I at least have this terrible relationship.
3. I dont want to be alone and its too hard to find a person to be with.
4. I dont like anyone so I might as well keep this bad relationship.
5. Life is too short I cannot build as meaningful a relationship with someone else in my remaining years.

Use constant little optimistic steps to break the cycle.

Explain to your partner what your needs are and what your limits are on their behavior. If they dont respect your needs or their behavior breaks the rules you should have clearly planned responses designed to help you seek counseling, limit your support for them, and finally begin to seek terminating the relationship. Be optimistic for yourself (and them to if they change for the good) Be faithful to yourself. Carry out your plans despite fear. If they disrespect your needs constantly then in short order you should be seeking a breakup and taking steps to secure yourself away from your partner. Just act like its an unchangable plan. You ll get a lot more respect from them and from yourself by not giving into fear and sticking to the planned separation/breakup steps. At some point they will realize you are serious and the inertia may change. if it doesnt then good ridance! Get yourself free and breath and start a new relationship with the same strategy. Keep repeating it until you find one that works.