If friends disappoint you over and over, that's in large part your own fault. Once someone has shown a tendency to be self-centered, you need to recognize that and take care of yourself; people aren't going to change simply because you want them to.
Friendship exists outside our modern economy of scarcity... It's not about apportioning vanishing resources of time and energy. Friendship is a blessed relic of the ancient economy of the gift, and the time freely given to people dear to you actually creates magical abundance.
Friends are an aid to the young, to guard them from error; to the elderly, to attend to their wants and to supplement their failing power of action; to those in the prime of life, to assist them to noble deeds.