This is the most critical and difficult shift you need to make. After a breakup, both people have a torrent of emotional needs: for reassurance, for comfort, for answers. Your first instinct might be to turn to the person who was, until recently, your primary source of support: your ex.
You must resist this impulse. Your ex is no longer responsible for your emotional needs.
Calling them to process your guilt, your nostalgia, or your loneliness is not only unfair to them (as it can be confusing and draining), but it is profoundly unhelpful for you. It keeps you tethered to the past and prevents you from developing the one skill you need most right now: the ability to soothe yourself.
This is where boundaries are non-negotiable. It doesn't necessarily mean cutting off all contact, especially if you have shared responsibilities like children or property. It does mean being ruthlessly honest about the difference between what you need to talk about (logistics) and what you want to talk about (feelings).
It's okay to miss your ex. Just don't ask them to fix that feeling for you. Let yourself miss them until, one day, you don't.