Most of the common reason of disappointment is your expectations from others. You expect things from every relationship. And whenever that expectation fails, you feel broken.
Do you expect something from yourself? Answer this question very honestly to yourself. Because a true answer will clarify many doubts. How many time you are able to meet up with your own expectations to yourself? How do you feel when you do not perform as per your expectations? Probably hurt and pain are there but no grudges against your own self. It is easy to let go because you know the reason why you failed in meeting up the expectations.
When same situation happens involving others you do not try to find out the reason but you start suspecting the intent of the person. You find it difficult to let go which causes emotional stress.
Humans are fallible and no one is perfect. Mistakes and misses can happen from anyone. If you understand this and let go things then you will see there is no room for disappointments.
You do not have control over the expectations you have from others as you are not the person to take action. You must expect more and more from yourself where you act and perform. You have full control.
Fulfill your own expectations and enjoy the happiness.