Paul is describing what it looks like to take off the old self and put on the new. And he is blunt about bitterness: get rid of it.
Get rid of all bitterness, rage, anger. He treats bitterness like something to actively take out, not a mood to wait out. It does not just evaporate on its own. You have to set it down on purpose.
Then the fuel he gives for it: forgiving one another, just as God through Christ has forgiven you. The power to forgive does not come from the other person deserving it. It comes from remembering how much you have already been forgiven.
So the question shifts. Not have they earned it, but have I forgotten what I was forgiven? Forgiveness flows down from the forgiveness you have already received. Start there, and the grip loosens.