Poorly differentiated people have difficulty handling anxiety. As a result, they deal with their anxiety through their relationships because emotional fusion can temporarily reduce anxiety and restore a sense of identity and purpose.
That is why poorly differentiated people dive into fusion when they are highly anxious. Consequently they become increasingly dependent on their relationship and their partner--or avoid emotional contact all together.
This can spiral downward. Anything that threatens your relationship creates even greater anxiety because it also threatens your capacity to cope with other challenges. As you go through repeated episodes of anxiety, you find yourself spending ever more energy trying to reduce anxiety.
In other words, you end up trying to control both your relationship and your partner in order to get control of yourself.
At this point it is important to underscore a new and important aspect to remember about differentiation. Differentiation is the ability to soothe your own anxiety and to resist being infected with other people's anxiety. Anxious and poorly differentiated people pass anxiety back and forth like a virus.
Differentiated people can modulate their thoughts and emotions when their anxiety start to run wild, and have their judgement prevail. They can chose to have feelings and intuitions govern their behavior on reason and intellect. The point is, they have a choice.