Simple Sentences:
* "I'm so disappointed."
* "That's a real let-down."
* "This isn't what I expected."
More Detailed Sentences:
* "I was really hoping for better results, but I'm disappointed."
* "The movie didn't live up to the hype, and I'm quite disappointed."
* "I'm disappointed that you didn't follow through on your promise."
Adding Emphasis:
* "I'm deeply disappointed in your behavior."
* "It's incredibly disappointing that this project fell apart."
* "My disappointment is immeasurable."
Using Figurative Language:
* "My hopes were crushed like a bug under a boot."
* "The disappointment is a bitter pill to swallow."
* "My heart sank like a stone when I heard the news."
You can also adjust the sentence to fit the specific situation. For example, you could express disappointment about a person's actions, a failed project, or an event that didn't go as planned.
Remember to choose the level of formality and emotion that fits the context.