I am so prone to codependent behavior, that it is very difficult for me to stay in a relationship where I have been an enabler without continuing to engage in enabling behaviors. I'm working at not creating new co-dependent relationships, but it is so ingrained in my nature to ignore my own feelings for the benefit of others and to tolerate boundary violations to avoid conflict and rejection, that for me, it has been a healthy step to be able to say "No. You don't get to keep doing that to me." To stay in a co-dependent relationship while trying to become less co-dependent would be like an alcoholic trying to recover from alcoholism while while working as a bar tender. Yeah, it would be great if I was that self-actualized, but I'm just not.
Seriously, if somebody gets doorslammed by me, I have bent over backwards and twisted myself up like a pretzel in hopes of being able to make things work. I know I sound like a stone-cold bitch, but I'm very loyal, accepting, and give people the benefit of the doubt well beyond the point where most people would. It's not any kind a virtue, it's just something I can't seem to help doing.
The difference between me and what you might consider normal is that I don't speak up as soon as I should, that I don't speak up as loudly as I should, and that when I make a decision to end a relationship it appears extremely abrupt and I often stick with that decision more rigidly than another type might.
Looking back at my own doorslamming experiences, I think what I've learned is how to recognize and avoid someone I should not enter into a relationship with. I'm also learning how to say no to doing things for or with people when I don't have the time, money, or energy to do them or maybe when I just don't want to or don't feel like it -- because those are valid reasons, too. As I learn to do that, I believe that doorslamming will become unnecessary behavior because my relationships will not get in that condition.
It seems like people want to focus on, what to me, is the wrong end of the problem. They want to invalidate the decision to end a relationship, when often the relationship should never have begun or the relationship has been allowed to go on too long or too dysfunctionally. It's this tendency toward invalidating ourselves and considering our own feelings and desires less valid than those of others that create this situation, more often than not. So further invalidation is not the solution, IMO.