I'm not sure if I understand your definition of fusion. I would see fusion at the far end of the spectrum of connectedness. A connection between a couple is a good thing. Fusion sounds like an extreme. So my analogy would be that connectedness is like weight gain and fusion is like morbid obesity.
In my parent's marriage my mom said that what got them through the financially difficult years is that one of them always managed to stay up while the other was down. So if they were "fused" wouldn't they have fed off the negative? Instead their connection allowed them to balance each other out.
Again I guess I would say that being connected is part of a long term relationship/marriage and is a very good part of it. But being fused seems like you're getting into more dangerous territory.
In long term relationships, it's very common for one person to depend on another for emotional support. You said that one of your parents would be up while the other was down, and this worked for them. How about if your father was the one that was usually down? Then your mother would feel the pressure to be "up" all the time, even if she didn't actually feel that way.
This also plays itself out in terms of self esteem. I used to compliment my wife's appearance all of the time, but she was constantly putting herself down. I started responding to her self put downs with compliments, but after a while, I got tired of trying to prop her up. It didn't work, so why bother. The same thing has happened with her anxiety. I've tried to calm her, but the cycles of self talk that go round and round in her head tend to crowd out my reassurances.
My attempts to prop her up have led me to loose respect for her. I don't try that hard to prop her up any more. I'm more inclined to just listen to her and say "I'm sorry you feel that way" or to just be honest and factual with her fears.
Another way to understand fusion is to see that it's opposite is not detachment, it is unattachment. If your reaction to being fused to your parents is to move to the other side of the country and never speak to them again, you are still fused. It's not clinging, but it's not aversion either.
Here is an analogy that I got from a book I'm reading called "Open to Desire" by Mark Epstein. Lets say you have a quarter in your hand. Make a closed fist. This is the grasping, clinging quality of fusion. You want that quarter so bad, you'll hurt anyone, including yourself, before you'll let it go. Now hold your hand palm up and open it. You still want it, it is still in your hand, but you are no longer grasping onto it. If you loose it, it's not the end of the world.
You can hold your partner lightly, in a way that does not consume them or cling desperately to them or overly rely on them to validate your self worth. That is differentiation.
SM
P.S. I think you will really like "Passionate Marriage". It is very challenging and thought provoking. Not your everyday self help book.
"If we will be quiet and ready enough, we shall find compensation in every disappointment." Henry David Thoreau
