A grievance is like a statement of case the party's counsel sends to the court. Written, static; facts and arguments; claim.
A resentment is like the trial itself: the out-loud presentation of the arguments and counterarguments; back and forth; cross-examination; dragging on and on; ill-tempered.
A grievance is fixed; a resentment is dynamic.
Of course, resentment is an activity that must be refrained from, as one would refrain from shoplifting or sexual impropriety.
The grievance may be a different matter. The facts might indeed be facts. The arguments and claims may be fair. But the grievance must be placed in the hands of God for either abandonment or pursuit. If it is pursued by God, i.e. redress is required, God will handle that. I need not track progress. Once I have posted it to God, it is no longer any of my business, and my job is to return to benevolent regard for the welfare of others.