I read the following verse which I am familiar with:
For if ye forgive men their trespasses, your heavenly Father will also forgive you. Mathew 6:14
Why is it so hard to forgive others who trespass against us when we feel offended, deeply hurt and betrayed?
The "reason" it is hard, is simply because of our sin nature. Our sin nature says our sin is not as bad as another's sin. It says the little "white" lie I told is not as bad as the money someone stole from the bank.
We do not see how we hurt others, so when others hurt us we feel offended, hurt, and betrayed. That's why its hard. When our eyes are open to how even our little sin effects others, then forgiveness becomes much 'easier'. Point being - forgiveness being hard is an issue of pride.
I'll say this very clearly. It is impossible to forgive others, and I do mean honestly forgive them, until you understand the forgiveness God gives you. Often times the ones that struggle with forgiveness are the ones who do not find their sin toward God 'all that bad'. They generally believe they are 'better' than most.
There are a few parables and stories that give us this insight. The one that really settles it in for me is in Luke 7.
Luk 7:36-50
One of the Pharisees asked him to eat with him, and he went into the Pharisee’s house and reclined at table. And behold, a woman of the city, who was a sinner, when she learned that he was reclining at table in the Pharisee’s house, brought an alabaster flask of ointment, and standing behind him at his feet, weeping, she began to wet his feet with her tears and wiped them with the hair of her head and kissed his feet and anointed them with the ointment. Now when the Pharisee who had invited him saw this, he said to himself, “If this man were a prophet, he would have known who and what sort of woman this is who is touching him, for she is a sinner.” And Jesus answering said to him, “Simon, I have something to say to you.” And he answered, “Say it, Teacher.” “A certain moneylender had two debtors. One owed five hundred denarii, and the other fifty. When they could not pay, he cancelled the debt of both. Now which of them will love him more?” Simon answered, “The one, I suppose, for whom he cancelled the larger debt.” And he said to him, “You have judged rightly.” Then turning toward the woman he said to Simon, “Do you see this woman? I entered your house; you gave me no water for my feet, but she has wet my feet with her tears and wiped them with her hair. You gave me no kiss, but from the time I came in she has not ceased to kiss my feet. You did not anoint my head with oil, but she has anointed my feet with ointment. Therefore I tell you, her sins, which are many, are forgiven—for she loved much. But he who is forgiven little, loves little.” And he said to her, “Your sins are forgiven.” Then those who were at table with him began to say among themselves, “Who is this, who even forgives sins?” And he said to the woman, “Your faith has saved you; go in peace.”
How often I have heard people say "I'll forgive them, but I don't have to forget....." - that's not true forgiveness. Seriously, its not. Would God forgive in any other way than true forgiveness? Of course not. Does God forgive us, yet keep record of those things? I sure hope not.....
Psa 32:1-2
A Maskil of David.
Blessed is the one whose transgression is forgiven,
whose sin is covered.
Blessed is the man against whom the LORD counts no iniquity,
and in whose spirit there is no deceit.
That phrase "counts no" is;
châshab, khaw-shab'; a primitive root; properly, to plait or interpenetrate, i.e. (literally) to weave or (generally) to fabricate; figuratively, to plot or contrive (usually in a malicious sense); hence (from the mental effort) to think, regard, value, compute:—(make) account (of), conceive, consider, count, cunning (man, work, workman), devise, esteem, find out, forecast, hold, imagine, impute, invent, be like, mean, purpose, reckon(-ing be made), regard, think.
Here is another one;
Psa 103:8-14
The LORD is merciful and gracious,
slow to anger and abounding in steadfast love.
He will not always chide,
nor will he keep his anger forever.
He does not deal with us according to our sins,
nor repay us according to our iniquities.
For as high as the heavens are above the earth,
so great is his steadfast love toward those who fear him;
as far as the east is from the west,
so far does he remove our transgressions from us.
As a father shows compassion to his children,
so the LORD shows compassion to those who fear him.
For he knows our frame;
he remembers that we are dust.
This is what true forgiveness is. So, the reason it is hard is two fold. One, we do not think our sin is that bad. Two, we try to forgive without forgetting.