Resentment is a very caustic thought and emotion, perhaps one of the worst. It is hard to get rid of even after you think you have got rid of it. You can say that you forgive someone a hundred times and think you have got past it, but then something small brings everything up again.

Resentment that comes of personal slights and hurts is often the hardest to erase because the hurt might still be buried deep long after the action or word has taken effect. It's personal nature tends to make it that much more searing or humiliating.

As a Christian, it my duty ( and my desire)  to forgive and give up any resentments. As Paul says in Ephesians 4.31, 

'Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice.'

That doesn't leave a lot of room for resentment or envy or jealousy, which are all forms of bitterness or malice. They are a trio that always turn out for the worst.

Prayerful forgiveness and turning the matter over to God are certainly helpful. Surrendering wholly to the Lord gives you a much better chance of 'letting go.'

Being flawed as we are, it is always a work in progress.