Only 15 percent of musicians are naturally comfortable performing in public. If you're not one of those fortunate few, you'll need to practice performing. Look at it like 250 public performances will make you relaxed and the quicker you can get those 250 done, the better. For our purposes, "public" is a performance for any one who makes you nervous, makes you want to do well. Parents, friends, neighbors, teachers, church members, enemies - anyone at all except those people with whom you are completely comfortable. Take every opportunity you can get to play/sing in front of others, at home, at school, anywhere to get your 250 performances out of the way as soon as possible, when it doesn't really matter if you mess up. Then when it matters, you'll be rock-solid.
You've got to complete some songs to be a musician. Don't just keep playing 100 licks and excepts - regularly play entire pieces.