When a client complains to me about their partner, a question I will eventually ask is, "Why did you pick them?" Granted, having a partner who is abusive, unfaithful, addicted, emotionally unavailable, etc. hurts. And you have to process that hurt and hold them accountable for their behavior.
However, regardless of what is "wrong" with the other person, I truly believe that relationships are, for the most part, 50-50. Your partner may be behaving badly, but that still leaves the question, "Why did you pick them?" If you don't answer that, you may be doomed to repeat the pattern. It's not uncommon to see a man with an addicted wife fight to break free from the unhealthy relationship only to get with another addict. Or a woman married to a man who cheats on her, acknowledges the deception, breaks off the relationship, then begins dating another cheater.
I had one client who was only attracted to married men, but was constantly bemoaning the fact that her latest conquest would not leave his wife and marry her. Yet, that was part of his allure, that he was unavailable. It kept her from having to really commit to the relationship. When one of them actually left his wife she almost died in a panic! She had been raised by parents who were emotionally unavailable to her and that is what felt normal to her. So she replicated it in her adult relationships.
Sometimes it is a matter of not picking. I've seen people who simply take whoever is interested in them. That too is something to ponder. Why do they not make a conscious choice? Do they think they don't deserve to? Do they believe they won't be able to find anyone else? Then we need to look at that.
If you're in a bad relationship, when you figure out what the other person's problem is, I strongly recommend that you take a good look in the mirror as well. If we don't learn from the past, we are doomed to repeat it.