Children are never sure of their abilities. A public attack on intelligence hits their most vulnerable spot. Virulent criticism doesn't motivate children to improve; on the contrary, it ruins their initiative.
When teachers are at their best they display a common orientation: they do not believe in the power of pontification. They neither preach nor moralize. They give no guilt and demand no promises... are not preoccupied with the child's past history or distant future, they deal with the present. What matters to them is the here and now of the child in distress.
Acknowledging experience and reflecting feelings are helpful interpersonal skills. However, they are not tricks or gimmicks. Nor can they be used mechanically. They are helpful only within a context of concern and respect. In human relations the agents of help are never solely the techniques, but the person who employs them. Without compassion and authenticity, the techniques fail.
First, of all do not deny your teenager's perception. Do not argue with his experience. Do not disown his feelings. Specifically, do not try to convince him that what he sees or hears or feels or senses is not so.