No one alive is immune to disappointment except the dead. Disappointment is strangely a part of life as it gets to teach us not to trust and depend on other humans who are fallible. Humans are prone to make mistakes, consciously or unconsciously. If we have this at the back of our mind, it would to cushion the hurt we feel when we are disappointed.

that's right. accept it like a part of life, let it come and go, don't care about it too much.