I held resentment towards my husband for many years for doing something that resulted in all my camera equipment being stolen.
I was having a sculpture opening.
Unknown to me, he thought it would be a great idea to take photos of the sculptures with people looking at and talking about them. He grabbed my camera gear, put it in his old car and drove to the gallery. But by the time he got there he felt it would be too much effort to schlep it in, so left it on the back seat. It was an area notorious for break, entry and theft - and that's exactly what happened.
I held on to my anger and resentment for 15 years, until I finally saved up enough to replace the equipment. (He was always too poor to afford that.)
Amazingly, as soon as I had the gear I no longer felt even a shred of anger. All that emotion had suddenly become irrelevant, a thing of the past.
I did learn to make a rule with him that he is not to touch or use anything of mine unless he asks first. And to ensure that, I keep it locked up.
Scores of years later, he was diagnosed with severe ADHD - impulsiveness, forgetfulness, lack of thinking skills and lack of working memory. He always meant well, but his neurological condition let him down countless times a day. As soon as he started taking Ritalin, he became like a normal person, much easier to live with.
So much of dealing with resentment depends upon the individual situation.
We are usually powerless to do much about others other than having a conversation with the intention of sorting things out. That takes courage, honesty, tact, empathy and persona; responsibility. Sometimes it's impossible to fix some things with some people.
In situations like this the old Serenity Prayer often helps:
May I develop the serenity to accept that which cannot be changed; courage to change that which can be changed, and wisdom to know the one from the other.
The simplest answer to this is that we are powerless to change others, but we can always change ourselves.
Regular mindfulness meditation is a great help for developing equanimity. There are many kinds. Personally I like Vipassana best because it is just bare bones observation of whatever arises in the present moment - observing without clinging - allowing sense perceptions to arise and pass away - for about an hour a day - or as much as you can make time for.
It also helps to start with examining within oneself.
If feeling anger, go for a vigorous walk or run, dance or do some hard labour; this uses up the adrenaline in a healthy way and prevents it from eroding your immune system.
Think back to the incidents that led to the resentment.
What happened?
What role did you play?
Were you a victim?
If so, how did you get into that situation?
How were your boundaries?
What prevented you from stopping that person from doing whatever they did? Or walking away?
Could it have been prevented or resolved with better communication skills?
Or greater precautions?
See how much you can learn from your part in the situation.
Make a list of the skills you might need to learn in order to relate easily and well with others, part also realistically. Research how, where, when and with whom you can learn and practice them.
When we have greater skills, the effects of others in the past can fall away?
Was there some specific loss?
How could you rebuild what you need for yourself and perhaps those around you?
How does your resentment affect you?
Is it stressful? Does it trigger fantasies of revenge (grotesque, impotent and not even possible)?
Does it affect your personality and your internal relationship with yourself?
Does it cause you to criticize or judge yourself or distrust your judgement? If so, how does this affect your other relationships?
Do you fully like and accept yourself as you are?
After processing all this and learning as much as one can from the situation, then it's time to let go of the resentment.
The easiest way is, whenever memories, images or thoughts of that person arise, turn your mind to something else - what's happening in your environment right now, a job that needs doing, some entertainment, time with a loved one, or maybe a walk in a beautiful place.
