Others say you should focus more on doing something positive rather than giving up something negative. I think you can do both at once, by which I mean focusing on forgiveness.
Forgiveness is a positive act; giving up something is a negative one. By "negative" I don't mean bad, I just mean that you remove something that was there. In this case, a debt you are owed.
We know that the Bible refers to our sins using the metaphor of debt, that when we harm others we owe them for it, that those who have harmed us owe us. One translation of that section of the Lord's Prayer is "forgive us our debts as we forgive our debtors."
I find this very useful, not just because I desperately want my debts forgiven, but also because it gives me a helpful visual aid for those who have sinned against me.
Yes, the old Parker Brothers Game of Life Promissory Note, which paves the road to penury and lands you in the Poor Farm at the end. Let's say this represents the grievance committed against me by another. Like, perhaps, vicious gossip spread by someone who meant to get me in trouble.
I decide I will obey Jesus and forgive this person. Not trying to make excuses for her; it could have been a completely evil act, and I still am under orders to forgive her. So there's my promissory note, what she would owe me on the Day of Judgment were I to hold on to the memory like an Irish elephant. But I determine that I am going to forgive this person.
The problem with forgiveness is that I can do it, and then find myself getting mad all over again when the incident comes to mind. C. S. Lewis wrote about this, about not only forgiving once but every time the memory angers us.*
By burning the note---usually just mentally; I don't want to run through board game supplies---I realize that I have to let it go, because it was a bearer bond and I destroyed it. I can no longer cash in that note. It's gone.
In that way, I can make something into ashes, and make it into something good, all at the same time.
* I forget which book that was in---I may make it a Lenten mission to find out.