If resentment is persisting, it is valued. One may not enjoy it, but that is a different matter.
The way to discover its value is to imagine being free of it. If there is any resistance (and there will be resistance to being free of resentment, if it is being held onto), the fear is likely to be this: 'if I stop resenting, I'll be defenceless, and the world will be able to do what it wants to me'.
It is in this that the fallacy lies: that to resent is to defend, and to forgive (which is the withdrawal of judgement) is to leave one defenceless.
The truth is the reverse: when I attack, mentally (which is what resentment is), I feel under attack; when I forgive (as defined above), I feel invulnerable (which I indeed am, being, as I am, spirit, not a body).
If resentment is persisting, that is the question: what value do I still see in it? I cannot fool myself about values.