take the wind out of one's sails (idea)

When you take the wind out of somebody's sails, you have stopped their momentum and dampened their spirits. This is not usually passive, like accidentally disappointing someone. When you take the wind out of somebody's sails, you are usually pointing out a serious problem with their plans, or using your authority to say that they need to stop what they're doing.

This implies that the person was emotionally involved with their plans, or was caught up in the rush of bringing them to fruition. The plan seemed to be going smoothly until you introduced reality into the picture.