Self-confidence is extremely important in any creative field. The fact that you are producing something that comes from your own will power can be both invigorating and frightening. Many of us artists often connect our self worth to the worth of our product, which is a mistake. The product cannot "be" its producer in that sense. If I were to miss a note on an excerpt, it doesn't mean that I myself am a missed note, my action was the missed note. Just because I failed a test doesn't mean that I am a failure, the attempt was failed. Disappointments are inevitable in life on any level, and in most cases they occur outside the person. When I first thought I was a failure, it was because of an outside influence that made me think I was. Be careful how you let circumstances effect your optimism and self-confidence. As long as you have the will to do something, that "will" will inspire ability and that ability will inspire action towards the goal.
Now, where most of artists find themselves, is the point of circulating self-doubt: I make a mistake, which makes me fear making another one, which makes me scared of taking another chance because think I am not capable of not repeating the mistake, which makes me think that all I can do is make a mistake. You find this in any form like constant sadness, fear of hurting someone, fear of being hurt yourself. These situations influence our view of ourselves, our "ego", in negative ways if we view them as being a statement of who we are. We can change what we do if we have the will to make those changes. As long as we give into the despair of the results, it will deter us from making a positive change towards better or more desired results.
In most cases, especially now, our ego has already formed, or been formed, to show us a false interpretation of who we really are. How many people do we know where they say they suck at something, but everyone around them knows that they are amazing at it? This is where we find ourselves, sometimes. People could've been told for years that they would amount to nothing, but if they got rid of that way of thinking, they realized it wasn't true in the first place. They didn't "turn out great", they were already great! Simply put, get rid of the negative ego and swap everything it tells you around to a positive. If it says, I'll never hit a right note, say I'll always hit a right note! This retrains your mind and ego to return to a truthful portrayal of who you are. I can't say that it is easy, because it's hard to believe something other than that which your used to, or that which is different than your own values or beliefs, but if you can open yourself to those differences and look objectively at them, you will see if they are indeed true.
Stay positive despite what it may look like on the outside. As long as you maintain that inside you, you can effect the circumstances in positive ways. As long as you yield to the negative you will be either remain stagnant or get worse. Don't! Be positive and grow in that! Let that inform you on what changes need to be made and what statements to believe. Get rid of the old, depressive ego and grow a new one! In the end, "believe in yourself, have faith in yourself"- Bruce Lee