Endings give people permission to behave in ways they would never otherwise consider, because whatever was holding them in place is gone. What remains is how you treat someone you once cared about, which is now entirely up to you and visible to everybody. Being decent afterward is hard, and it is the part you will be able to live with later. The hurt is real and it does not license anything.

Say nothing about them you would not say with them in the room. Return what is theirs. Let the ending be sad rather than ugly, since that much is still yours to decide.