What a disappointment.
What a bummer.
You let me down.
I messed up.
All that for nothing.
All my effort went down the drain.
The contest was a letdown.
The talk was very disappointing.
The movie was sort of disappointing.
I feel depressed for what you have done.
The service here is disappointing.
All my work was in vain.
A: Charlotte, have you had your supper?
B: No, I don't want to eat anything.
A: Why? Don't you feel well?
B: I'm down in spirits.
A: What's up?
B: My manager jumped on me for my mistake today.
A: You must not feel depressed about such a trivial thing.
B: I think I'm too clumsy. I can do nothing well.
A: You'd better shape up if you want to get the job done.
B: But I doubt myself.
A: Cheer up. Don't let me down. We all make mistakes and that is life.