As a tennis professional, I am always trying ways to teach players how to have confidence. You can say all day that you want your student to "Be Confident," but what exactly does that mean?
To me, the connection is made between looking at the ball as you are near the point of contact and having confidence that the ball will go where you want it to go without "looking" where you are hitting it. Once a player masters this, I believe he or she will always have confidence. Most players hit balls so "off center" because as the ball gets nearer to the contact point the average player will allow doubt to enter the picture and try to look where they want to hit the ball. I believe this causes poor hits and stray shots. Also, a player who cannot make quick decisions and then trust those decisions has a difficult time playing tennis. Always keep your head still as you go to the point of contact.
Always assume that you are a good player, that your shots always go where you want them to go and that you cannot make calls on your opponents side of the net, thus a great player will always assume that their shots are in! If you don't watch where your ball goes, but instead, you focus on the racket face of your opponent, it will allow you more time to get ready for the shot that comes back to you and it will better enable you to read the shot.
If you are looking for immediate satisfaction as you hit the ball, you will probably not reach your potential. Teaching this to players can also help them with life decisions. Get all the information you can before making decisions and trust your decisions once you make them.
Of course, this is not a quick fix, and there are many different aspects of tennis that can help improve confidence based on an individual's game. It is, however, one element where easy adjustments can help most players gain some consistency and confidence.
Good luck on the courts!