"I expected more" should be received as an encouragement, more than a criticism.
Somebody thinks you can do even better! Somebody thinks you can deliver move value, more work, more art.
Sure, it hurts, especially if you know it is true. Then the "buts" arrive: But I had multiple assignments, but there wasn't enough time, but there wasn't enough budget, but I didn't understand, but I was sick, etc.
It is important not to belabor what was just evaluated: you can never go back and redo the effort (though you can bring new effort and redo the project.) The key is to know that somebody believes you can do more, next time!