The rest you learn by doing. If the Lord has given you the ability to communicate the Gospel to others through music, you really don't need to ask any more questions. Feelings of unworthiness come and go, but your position in Christ never changes. Use every ability you have been given to express Him to others. You'll change the world and find the fulfillment of your heart's desires.
Most people go to a mirror several times daily as a part of grooming.
We want to know what we look like. It isn't something we can know without a mirror. We need to go there frequently so that our true identity becomes something that we are not just educated about, but that we walk in as a reality daily. Who and what we are is not something we can know or maintain knowlege of without His word. It is a temptation to judge our worth as musicians, ministers and even as people by the reactions and assessments of other people.