I am Catholic and I struggled with this one too. I've read a lot, and that helped me through this. For example, following are two excerpts from a Catholic author and psychotherapist Gregory Popcak, in his book God help me! These people are driving me nuts. (I still chuckle at the title, since I related to it so much at the time (due to AH):
"Scripture says, "You are my friends, if you keep my commandments" (John 15 14, italics mine). In a sense, this is the scriptural basis for the limits on tolerance. Jesus was saying, "I love you more than you could ever know but if you want to be in a relationship with me, then you need to act in a certain way toward me. If you do not, I will withdraw from you, and if you continue to act offensively toward me, you may lose my friendship entirely." Mr. Popcak continues to elaborate on how he made these conclusions referencing more of scripture.
...on being co-dependent:...
"While Jesus offers himself completely to those who will respond to him, to those that will offer themselves in exchange for all he has to offer them, he does not spend himself unwisely on those who cannot or will not reciprocate. To put it in scriptural terms, he does not cast pearls before swine. Jesus endured indignity, suffering, and humiliation only when doing so was absolutely necessary to fulfilling his mission, and then he did it with grace and fortitude. By contrast, the true co-dependent is unable to distinguish between necessary suffering which results from efforts to become or help someone become the person God intended and unnecessary suffering which perpetuates a persons pathology or sin and destroys the person who is trying to help." Once again, the author elaborates more to make his point more clear and lists examples of when Jesus avoided suffering, etc.
I personally came to conclude that my relationship had gotten to the point that I was perpetuating my AH's pathology or sin by remaining in the relationship. I had a priest/friend tell me that perpetuating someone else's pathology can amount to sin on my part...and that made sense, too...it really helped me to see things differently now that I was seeing a bigger picture.