When someone says "I can't imagine life with you", they're actually saying "I can't imagine life *after losing* you".

Having never had is not the same as having and losing.

Losing someone isn't some reset button that magically changes everything back to the way it was before you had them. You carry those experiences, memories, and changes you made during that time.

Then there are degrees of attachment and integration that will affect the aftermath of a breakup or divorce.