If you think you’re approaching the high point, but then realize it’s really the low point, that’s an anticlimax — a moment when excitement quickly changes to disappointment.
In a movie or book, a climax is the point where, after a long buildup, everything gets really intense and dramatic. But if there’s a lot of buildup and then suddenly something really boring happens, that’s an anticlimax. Like waiting all day to see fireworks, but then nobody has any matches. It can also mean discussing something important like war or art, and then someone interrupts to talk about candy. Talk about a letdown.