Before you reject that you may fall in this camp, please note that NOBODY is able to perfectly conform to the ideal of having a well-differentiated "self." Why? We all grow up in family systems or belong to groups where there is a groupthink mentality of some form or another. We are all influenced to some degree by the way others think, feel, and act. That is what it is to have a relationship. However, when people have a higher (remember the continuum or sliding scale that people function on--you change how you function based on any given situation) differentiation of self, then they recognize they need others, but they tend to depend less on other people's acceptance and approval. This means that they are able to put their feelings on hold before they make a decision about important family and social issues (in other words they don't morph or take on the attitude and beliefs of those around them without first thinking through them). When people have a higher level of self differentiation, then they are more able to stay calm, level-headed, unemotional when they are in a conflict, rejected or criticized. When they find themselves in a conflict, rejected, or criticized they are able to act in the best interest for themselves or a group by staying calm so they can think through a situation based on facts rather than emotions. Whatever they decide matches both what they say and do.
On the scale of self differentiation then--people that are able to think through a situation, and not just react based on their emotions or the emotions of a group would be the ones with a higher differentiation of self.
Not to confuse matters....your level of self differentiation changes based on the situation. Everyone has two types of self differentiation: basic and functional. Your basic differentiation of self level is based solely on the level of your family of origin. Your functional differentiation of self level is based on skills you have learned to better cope under stress.
For example, if your parents have say a level of 40 on a 100 point scale (by the way--there are no actual scales that any professional measures you on when talking about this concept), then you will have a level of a 40. The only way your basic level EVER changes is through hard work re-learning family dynamics and actually putting in cognitive and behavioral changes into place. The more you are able to logically think through and react to a situation, the better you will be able to handle ANY situation or relationship.
In fact in order to have healthier relationships this is how you go about making your first changes, because now you will be able to control your emotions so you are able to be proactive and not reactive. In other words you go from a victim or bully mentality to one of empowerment. And this change is after years of work at re-patterning neural pathways in your brain. It is possible, but not probable that your basic level will change. HOWEVER, we all have another blessed level of differentiation of self and that is functional.
Your functional level is the one that you tend to operate at in your job and with friends when you are under "NO" stress. Meaning, if you get a deadline at work, or you have a tiff with a friend/neighbor/colleague...., then you tend to revert back to your basic level of differentiation of self. If you wonder why you do so well at work, but then your personal relationships suffer it is because you have two different ways of interacting with others. Becoming aware that your basic level of self differentiation is lower on the scale is the first step toward being able to operate consistently at your higher functional self differentiation level.
I will leave you with one last thing to think about...we just went over what differentiation of self means, and you probably put yourself on a sliding scale between having a poor or well differentiation of self. You've found out that most people (you included) have a higher functional level of self differentiation that they tend to operate at on a daily basis at work and with friends, but when subject to stress (conflict, criticism, rejection) they will find themselves reverting back to their basic level--the same level that their family of origin gave them. Our functional level is raised by learning coping skills so we are able to rationally think through things before making a decision, rather than letting our emotions dictate what we say and do. So....in regards to relationships, and if you are trying to find the love of your life. Guess what? You attract and mate with people of the same BASIC level of self differentiation. You may also read that you attract a mate with the same level of maturity. This is often why basic levels stay the same in families. The only way you attract someone with a higher basic level is if you have done the hard work with yourself of raising your functional level (through learning and putting coping skills into practice on a daily basis EVEN when you are not under stress), and you are able to be mindful and rational with your decision-making. This is one of the reasons why in co-dependent relationships where there is substance abuse or mental health issues....if only one person either the one with the problem or the unaffected partner decides to make an active change and operate at their functional level even when in conflict--the relationship falls apart.
Therefore, if you are in a relationship or looking for one we go back to the same basic point. The only way you have healthier relationships is if you are aware: (1) the only person or thing you control is yourself and your feelings, thoughts, actions, and (2) the only way you change this is through learning and putting these coping skills into practice! Pretty cool! You can and will have the relationships you desire if you're able to understand and implement these changes in your life!