Be kind to one another, tenderhearted, forgiving each other, just as God also in Christ forgave you.
Paul puts three things together, and they are not equal in size. Be kind. Be tenderhearted. Forgive, just as God in Christ forgave you.
The order matters. Kindness and tenderness come first, because a hard heart cannot forgive. It can only keep a complaint. Forgiveness is what happens when someone has stopped defending themselves long enough to remember what they were given.
Notice how specific Paul is about the model. Not “as you would like to be treated”, which is a decent rule of thumb. Something specific happened, at a specific cost, and it was done for you.
Forgiveness is always expensive, and it was expensive for him too. If it had cost nothing it would not have been forgiveness at all. It would have been indifference. You are not being asked to feel warmly about the person who hurt you. You are being asked to release a debt, knowing yours was released first.