What is the most difficult adjustment in the aftermath of relationship breakup?

Depending upon the length and quality of any relationship, many aspects are going to come up. Insofar as you continuing on and breaking the throes of disappointment, sadness, etc., what are you going to miss the most about the other person's value, contribution (e.g., emotional/mental security, friend/ confidant, being able to be there for you when needed, partnership in fun, adventure; level of nurturing during stressful situations (such as ability to be patient and attentive to your health challenges) to the level of living your new life?