Hazrat Inayat Khan answers a question in a collective class:

Q.: I am disgusted with life, with my fellowmen, I always try to be friendly to all, but I receive nothing but unkindness. Life seems not worthwhile to me. What shall I do?
A.: Why must you have no confidence in your fellowmen, if one or two persons have disappointed you? How many persons have disappointed you?
Q.: I always have been disappointed in my life.
A.: How many, perhaps a few hundred persons? But how many millions of people there are in the world. Can you compare with the hundred persons the millions and millions that live on the earth? Can you not have a hope that if you are disappointed by one hundred persons, perhaps with ten hundred you could have another experience? There is life before you. Even if it were that all your life through you had disappointments, still if I were you, I would be hopeful and expecting that even to the end of my life I will meet someone in whom I can have confidence. There is no reason that you should lose your confidence.
Q.: Is there any concentration of practice [for this]?
A.: The best practice is to forget and to forgive. Any wrong that anyone has done, to forget and to forgive. Yes, you do not need to follow that person, if you do not want to, that person who has once disappointed you. But you can forget. In order to erase that impression that has brought about in you that lack of confidence, you can erase it.
Q.: How to erase that feeling of bitterness?
A.: But you must see it in this way: that when a person is feverish, that every sweet, sour, or salt thing, anything one tastes, it is tasteless, disagreeable. Therefore all those who deserve your sympathy, friendship, they look like it [i.e. tasteless, disagreeable], because you have that bitterness in you. Therefore it is much better to try and forget that bitter feeling. Therefore suggest to yourself: I no longer have any bitterness in my heart, I no longer blame anyone, and I no longer have anything against anyone. I have forgotten, I have forgiven all that anyone has done to me, except good things.
A person who expects disappointment will get it from the best person in the world. A person who says: everybody is unjust to me, let him go to the throne of God, from there he will come with the same blame. He sees nothing but injustice. When one begins to see wrong, everything is wrong. To the one who sees in the right light, everything is right.