You need not experience extraordinary suffering or be wronged in an unusually grievous way to feel the strong, seemingly unstoppable pull toward resentment. All you need to do is live a little in this fallen world. Before long you're given a good solid reason to resent someone. Often someone quite close to you. Family member, spouse, parent, long-time friend. It feels impossible to love that person.
What causes such bitterness? How do our hearts become so immovably deadened toward that person?
They wronged you, so you resent them. They hurt you. They did what they should never have done. Or didn't do what they should have done. And you bear the wounds.
But what's the reason beneath the reason? The fundamental reason is your God-given sense of justice---itself a good thing. You have been wronged, and you, created in God's image and therefore with a rightly functioning sense of justice, of fairness, cry out that justice be done. The playing field must be leveled. Fairness demands it.
The trouble is that as a law-abiding citizen you know you can't do something physically to them, as you may wish to (let's just be honest here). And as a Christian you know you can't verbally or publicly do something to them (perhaps simply because you would rather keep your reputation and leave them alone than exact revenge and lose your reputation; the greater idol outweighs the lesser).
So what happens? Where does a gospel-vacuous heart go in such a case? Instead of doing something externally to harm them you do something internally to harm them. You harbor bitterness. This is the psychology of resentment. You exercise emotional punishment toward them internally when actual punishment can't be exercised externally. You set up a law-court in your heart, since an actual law-court is unfeasible.
But here's what happens. The bitterness you harbor, the emotional punishment you exact in your heart, has precisely the opposite effect, over time, than you think. Bitterness does nothing to the offender, while it quietly destroys the offended. Resentment kills, hollows out the resenter, not the resented.
Soak in These Realities
How then do we conquer bitterness?
By soaking in two realities.
First, God is the judge. He has a law court. A real law-court. And one day every person on the face of the earth who is not in Christ will be the defendant. The Bible even says that Christians one day will themselves assist God in judging the world, even judging the angels (1 Cor 6:2-3). Fairness, justice, and righting of wrongs is gloriously inevitable. Your day of judging your offender is coming. But it is not today. You will take up the gavel. Just not today. If you seek to exact premature judgment, you destroy yourself.
Second, and most crucially, you have offended God. And continue to offend him, in a hundred ways you know and a thousand you do not, every day. But God didn't harbor bitterness against you. He didn't resent you. He sent his Son for you. God laid down every reason to resent you. Having been forgiven, how in the world could we resent another? Here's C. S. Lewis, "On Forgiveness":
To be a Christian means to forgive the inexcusable, because God has forgiven the inexcusable in you. This is hard. It is perhaps not so hard to forgive a single person great injury. But to forgive the incessant provocations of daily life---to keep on forgiving the bossy mother-in-law, the bullying husband, the nagging wife, the selfish daughter, the deceitful son---how can we do it? Only, I think, by remembering where we stand, by meaning our words when we say our prayers each night "forgive us our trespasses as we forgive those that trespass against us." We are offered forgiveness on no other terms. To refuse is to refuse God's mercy for ourselves.