do not let resentment teach the room

Paul does not introduce forgiveness as a feeling you have to produce. He lists it among things you put on - compassion, kindness, humility, meekness, patience - like clothing you choose before you leave the house.
Heiser points out that the whole section is aimed at the treatment of other people. And the measure Paul gives is not proportional to the offence: "as the Lord has forgiven you, so you also must forgive." Then verse 15 says where this happens - the peace of Christ ruling in hearts, "to which indeed you were called in one body." Resentment is not a private matter. It teaches the room.
Scripture links:
- Colossians 3:12-13 - "Put on then, as God's chosen ones... bearing with one another and, if one has a complaint against another, forgiving each other; as the Lord has forgiven you, so you also must forgive."
- Colossians 3:14-15 - "And above all these put on love... And let the peace of Christ rule in your hearts, to which indeed you were called in one body."
Someone is learning what grace looks like from how you handle being wronged.