Gratitude for a relationship that ended badly is a late arrival. It shows up years afterwards, usually attached to something small — a way of chopping an onion, a band you would never have listened to, the habit of ringing your mother on Sundays.
Those came from somebody. They stayed after the person left, and they are yours now.
Ease is a different matter, and it is not the entry requirement. Plenty of people have opened the door to somebody new while still finding the leaving hard. The two aren't sequenced.
What actually has to happen is smaller: enough room in an evening that a new person could occupy some of it without being compared, all night, to a ghost.