Strength through trust in God
One might deepen his understanding of bitachon (trust in God) and internalize its basic principles. There is great comfort and tranquility in the knowledge that the transformative events in a person's life occur only with the approval of God (and that everything that God does - whether or not we humans can see it - is for the best - see this essay for greater detail on this question).
If you could see the person who hurt you in that light, as a simple messenger of God (who, like all of us, will be held personally accountable for any errors) you could more easily forgive him. See II Samuel, 16; 10 were David, escaping from his rebellious son, is attacked and cursed by Shimi ben Geira (a great Torah scholar and, according to tradition, Shlomo's teacher). Urged to take revenge, David refuses, saying: "...for God said to him 'curse David' and who can say to him (Shimi) 'why are you doing this?'" In other words, David saw the hand of God in this insulting attack and felt it futile to blame the messenger.
The Talmud (Tractate Brochos 33b) says that "everything is in the hands of Heaven except the fear of Heaven." Take a quick look at Genesis 50; 19 and 20 where Joseph assures his brothers that even if he wanted to take revenge on them (for having sold him into slaver so many years before) he wouldn't be able to harm them if it was God's will that they not be harmed. That power, no human being has.
The joy of letting it go
Focus on the great and lofty spiritual (and emotional) level you can achieve by "letting it go by." The Gemara writes the following:
"Those who are insulted but don't insult others; who hear their degradation but don't reply; who do (divine service) out of love and feel joy over suffering, of them the verse says 'and those who love Him are like the coming out of the sun in its strength' (meaning in both the next world and in this they will be very very great)."
The Chofetz Chaim, in one of his many essays, writes that there are three distinct levels of greatness mentioned here: those who don't insult others, those who don't reply even at the time they're being wrongly shamed and those who actually feel joy at the suffering (I assume the joy is a product of the knowledge that this suffering is accomplishing many important things).
Perhaps this third level is beyond our immediate grasp (although, wouldn't it be so nice to find joy in suffering?), but there is greatness in the first two levels as well. Thinking about the greatness that's within your reach might itself be an inspiration to "close the account" on your own.
Seeing God as master planner
First of all, let's assume (as I do) that God is just and all-knowing and that there's plan and purpose for everything that happens to a person (excepting our personal choices: those rest in our own hands). Where in this world a soul is "dropped" - and in what physical state it arrives - is planned. So says the Talmud (Niddah 16b), where an angel is said to announce: "...this (unborn) child, what will become of it? (will it be) poor or wealthy, strong or weak, foolish or wise...".
If someone, through his lifetime, endures more challenges than another person, then I think it could be because his purpose in this world is inextricably wrapped up in succeeding despite, or even through, the pain. The impact of such success is enormous (both on the victorious soul and on those who watch and are inspired). It could be that it's that impact that God's really after.
This is taken from an essay by Rabbi E.E. Dessler (a foremost Torah philosopher in the first half of the 20th Century) on the subject of "mazel elion." (see the Hebrew edition of his collected writings Michtav M'Eliyahu volume 4, page 101). There, Rabbi Dessler explains that some people are intended to use their lives to create a certain sanctification of God's name and that any method will suffice. Therefore, if, for example, they're poor (and their impact can be had by showing how holy a person can become despite poverty), then elevated prayer and other merits could "convince" God to give them, let's say, wealth to see if they can accomplish the same level of greatness through that medium.
A clarifying example: "Chava" might be a woman of whom God expects outstanding devotion and sensitivity to the needs of weak and vulnerable people around her. Should Chava commit too much time and energy to her own needs and spend her days planning extensive vacations and wardrobe additions (or even to aspects of Divine service that come naturally to her), she will fail to live up to the potential for which she was created. God might send, let's say, illness or poverty to Chava, providing her with the opportunity to achieve a greater sensitivity to the ill and poor in general and, if she â€œwakes upâ€ in time, Chava will get the message and refocus her priorities.
But Chava also could also independently arouse her moral instinct and sensitize herself to the needs of others. She would thereby be achieving the greatness that God had always wanted of her but without the need of an unpleasant reminder.
With this, we can perhaps understand at least one aspect of the suffering of this world: could not these events - as inexplicable and undeserved as they seem - not be a call to somehow re-examine and refine one's inner core; to grow in some deep and meaningful way? I will suggest that this re-examination and growth could well become the happiest and most fulfilling experience of Chava's life.
(There are other people, writes Rabbi Dessler, whose destiny is to accomplish greatness in one way only...through poverty or pain, for instance. In those cases, no amount of merit will change the reality. But these souls are rare and usually particularly exalted).
So even if a child born to a difficult life doesn't enjoy the kind of childhood that we think all kids deserve, he might nevertheless reach sublime heights and great successes through the way he responds and grows. That's a reasonable and yet significant hope.
Remember: you CAN succeed.
I also believe that anyone who's ever faced a test has also been given the resources to succeed. See Genesis 4; 7, which, according to many commentaries, translates: "Isn't it true that if you correct your actions you will succeed and if you don't correct your actions, sin crouches at the door (i.e., your evil inclination waits to claim you for sin)..." From here our sages learn that no one is given a test that he can't pass...while difficult things can happen TO us, our actual success (in God's eyes) lies in our own hands.
"Unanswered" prayers - and the hope they represent
A century ago, many Jewish communities of eastern Europe were being destroyed by rampant assimilation. Marxism, anarchy, nationalism (in many flavors) and countless ideologies competed to replace religion in the minds and hearts of young Jews. Many, perhaps most, of the generation was permanently lost to Judaism and to their families.
Now how might the mothers and fathers of these young people have reacted? There is no doubt that many, many of them spilled oceans of tears in prayers for their dear children...but more often than not, their prayers seemingly remained unanswered.
Decades later, however, the Chazon Ish (Rabbi A.Y. Kareletz of Bnei Brak, Israel) commented that those prayers and tears surely did not go unanswered. In many cases, he observed, those prayers gave strength, not to the children, but to the grandchildren or even great grandchildren to allow them to find their tortuous way back to Judaism - often from unthinkably great spiritual distances.
Teshuva, returning to Judaism, is by no means a simple process. Often, logic would suggest that there's no point even starting...it's just to hard and too far to travel. But so many tens of thousands of Jews are making it. They're given strength, and often that strength is a gift from their ancestors.
Think, therefore, how much of an impact your prayers and tears can have on your own grandson's chances for success. Even if we don't see it right away, no prayer is ignored and every merit finds its rightful place.
A deep and hidden mission
Rashi (Gen. 5; 32) asks why Noah had to wait until his was 500 years old before having his first child even though others in his generation had their children much earlier. Rashi's answer is somewhat curious: so that Noah shouldn't have too many children who, if they turned out to be evil would be killed in the coming flood and that would cause their father anguish and, on the other hand, if they turned out good, there would be many of them and Noah would have to build many arks to hold them.
Now I'm sure you sense my problem with this Rashi: Which parent wouldn't give all he has to father (or mother) righteous children even if it would mean building hundreds of arks? Don't people happily sacrifice their financial well-being, their time and even their health for their children? Would Noah have resented the extra work?
I strongly believe that God, in withholding Noah's children, wasn't worried about Noah's reaction but, instead, was concerned for his time. Each human being has a unique role to play in this world; each requiring different temperaments and different resources. Sometimes, for some people, being bound by the demands and distractions of a large family can get in the way of fulfilling this role. Perhaps God wanted Noah to single-mindedly devote those many years of his "earlier" life to teaching and warning his neighbors...perhaps the many children he might have produced - great as they might have been - would have prevented him from fully realizing that mission.