What will Endure?
All the relationships in life are bound to fail and will not benefit us, except only one: the relationship between two people bonded purely and sincerely only for the Almighty Creator. Look at how the Companions' friendships endured, through the harshest of circumstances – persecution, wars, migrations, poverty and even death. That is because their bond was for the sake of Allah, not for fleeting common interests. This is the real relationship, as opposed to the fake or superficial relationships that many of us have with each other.
A relationship for the sake of Allah, as one scholar puts it, starts after death. Our relationship between each other for the Almighty Creator will never die. Even our death is the beginning, and we can benefit each other in this life and in the hereafter until the Day of Judgment.
Why? Because the glue between each other is the Almighty Allah, Who will never die, Who will never change and Who will never vanish. This makes the bond genuine and enduring. If the intention is real, the strength of the relationship will never deteriorate.
Admittedly, this is very rare. It needs two people who have a faith and willingness to generate this relationship only for the Almighty. The goal is so noble that they are willing to turn a blind eye to the mistakes, imperfections and shortcomings of each other. Hence, the relationship is not for beauty, body, money, status, or the superficial, for all these can change any time. The relationship is purely for love of Allah.
Should I Forgive?
There is a big difference between the friend who makes a mistake, lacks ability to deliver, or simply does not possess the qualities of a good person or friend. It is important to distinguish between them.
For example, if he does not bail you out financially because he lacks money himself, cannot be there physically to help you because he is living far away or cannot comfort you through your relationship problems because he is nursing a terminally ill family member, these are limitations on his part which we have to respect.
Additionally mistakes will always occur. None of us are perfect.
However, if the person lacks quality, that is different. For example, if the person does not have a good moral character or is intrinsically selfish. These characteristics are less obvious in everyday interaction. We only see certain aspects of a person at a time, usually only the good side. Under strain or different circumstances, a new facet is revealed. It is something that we have never seen before, but something that was always there all along. During times of need, the person shows an ugly, selfish or cruel side. It is this category of person that causes the most hurt.