So, what is differentiation? It's the opposite of symbiosis (connection as one - where codependency can arise...we will leave that to another day!)
Differentiation, the way we hold on to ourselves while staying connected to our partner. Differentiation requires the individual to be honest with themselves and partner, open to personal growth and the ability to express how they feel/think/desire while tolerating their partner doing the same. 
Differentiation facilitates authenticity and vulnerability, and resolving conflicts and accepting that you might not like each other at times!
Differentiation is something I talk to couples about from the beginning. It is interesting how often people become fearful of that change - within their partner or themselves- that holding integrity can create so much anxiety. In actual fact it can do quite the opposite! It can keep the passion alive, stop relationships becoming stale, break any repetitive cycle that's occurring with conflict (hostile/avoidance) and most importantly, ease re-traumatisation.
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Here are some tips to get you started with differentiation:
1. During a conflict or difficulty, try doing something that is 180 degrees different than what you have tried. 
2. Question yourself: How are the current relationship issues made worse by you, your attitude, your way of looking at issues.
3. Confront yourself, not your partner. See how it feels and how you contribute to issues. 
4. Ask yourself: Do I feel comfortable with my partner expressing their views, emotions or thoughts? •
See how it feels asking yourself these questions. If your differenation is strong in the relationship, you should be comfortable handing these over to your partner!