An unfulfilled expectation causes disappointment. Even though we know this, we continue having expectations. It is a habit. I have worked on this from the beginning, the middle and the end of the process.
In the beginning, I try to nail things down when making plans, so that as little as possible is left unsaid. Then in the middle, I try to manage my expectations by realizing that things can go wrong. At the end, I manage my emotions when things go wrong.
For managing your emotions, I recommend getting a book on cognitive behavioral therapy (CBT) and learning the technique. This book is highly rated on Amazon:
You have to practice this again and again. It gets easier over time.
©2016 Stephen L. Martin
Painting: Disappointment by Julius Leblanc Stewart