The cognitive stage:
Did the thing you are bothered by actually happen as you thought it did? Sometimes you are basing resentment on speculation, interpretation, extrapolation, or generalisation. Find someone cool-headed, bright, and analytical to take you down from the heady mountaintop of judgement to the flat plains of reason.
All resentments come from demands that have not been met. What is the demand? Why do you want the demand met?
Is the demand ego-based, i.e. a desire for some material or superficial and ultimately vain and unsatisfying commodity of the world? If so: recognise that you are angry because you have been deprived of something that is essentially of negligible worth.
If the demand is ordinary, e.g. for friendship, companionship, an occupation, etc., ask if you already have enough to be getting by with. Sometimes we have plenty of friends but get inordinately upset because one person goes off us. This is more about pride than need.
If we are genuinely being deprived of something, the job is to go to God and ask for God to find a way for the need to be fulfilled. Add this prayer: 'if there is anything I need to do, to bring Your plan to fruition, let me know.' Then let it go.
Regarding the behaviour of others: sometimes the behaviour is reasonable but is simply getting in your way. The solution here is to recognise that you are only one of many people on the planet and you are not entitled to have your way the whole time. This is entitlement.
Sometimes the behaviour of others is plain wrong: selfish, inconsiderate, thoughtless, wilfully destructive, negligent, reckless, etc. Even in those cases, all we have identified is that the individuals in question—like ourselves—have character defects. We have not identified an error on the part of creation, merely something that is getting in our way. The way to deal with reasonable or unreasonable behaviour on the part of others, therefore, is identical: we are not entitled to have the world refrain from interfering with our plans and designs, the world being an infinitely complex mechanism of interactions. We are not the centre of the world, and the rest of the world has no obligation to anticipate and work around our demands.
A further thought on the character defects of others: there is no reason on earth why others should be more virtuous than we are. I am not entitled to demand that others have character defects removed, even if a particular defect has been resolved in me; there are surely defects I still have that have been stripped from others, and it would be no more fair for others to demand that I be further along the path of spiritual development.
To sum up on others' defects: we accept them and their defects as they are. We do not perhaps care for the night as much as for the day; it gets in the way of our plans because we cannot see our way. We do not perhaps care for others' defects as much as for their virtues, but the principle is the same: they merely get in our way. They are no less natural and part of the truth of the world as night is part of the twenty-four hour cycle. The problem is not their existence; the problem is my demand that they not be there.
To sum up on my demands: almost everything I demand would not itself engender happiness in me were it to be delivered. If spiritually misaligned I would merely develop a new set of demands. Of this I have endless experience. Of those few demands whose non-fulfilment is a genuine bane, I can go to God and ask for any unmet needs to be met—in order that I can be useful to others—and in the meantime degrade the demand to a simple request, recognising that I can be happy under any circumstances provided that I ask God for courage to supplant fear, gratitude to supplant ingratitude, and humility to supplant entitlement.
The prayer stage:
Once I have adopted the proper mental attitude towards the situation, the emotion is likely still lingering. Prayer is required to cement the new attitude in place and to launch me forward onto a path of giving rather than concern about getting. The prayers are set out on the top of page 67 of the book 'Alcoholics Anonymous'. I will not reiterate them here; they are plain and not subject to interpretation. Note only that one prayer, with four elements, is set out explicitly; a couple of other prayers are implied by the results that we are told will flow from this prayer.
The action stage:
The action is two-fold: firstly, if I owe amends, I must make them. Often resentment lingers because it is the ego's way of displacing guilt: if I demonise you, it justifies retrospectively my bad behaviour towards you, because somehow you had it coming; secondly, I need to adopt the right role, under the guidance of God, in terms of my conduct towards you. Once I am acting right, it matters little whether you are. My side of the street is clean.
The above principles brook no exceptions. The practical application of them may require some ingenuity on occasion but the principles are universal.
There is no such thing as a justified resentment, because observation of even the most appalling behaviour does not necessitate or automatically give rise to a sustained emotional response. We are not puppets on strings; we choose our emotions by choosing our attitudes. We may observe that something is regrettable and feel the associated passing emotion on discovery of the regrettable fact, situation, event, or behaviour; but to linger on what is regrettable, pondering it, fulminating about it, contemplating it, and even meditating on it is both unnecessary and unwise. See, accept, respond if it is my duty or role to respond, and move on.