1. Body Language
Read "Presence" by Amy Cuddy and start holding your posture like a superhero. Your physical body position will signal a matching level of insecurity/confidence in your mind. When we play guitar, it's natural to slump forward, drop your head and slouch your shoulders. Try opening up, push your chest forward, breath into your belly and lift your face to greet the word. Just this alone will boost confidence because now you're visually and energetically presenting yourself to the world as a confident singer instead of a timid one.
2. Breath Deeper
Singing requires air and powerful singing requires lots of air pulled deep from the diaphragm. When we're nervous we often breath shallow in your upper chest because our belly feels tight with nerves. Start pulling the air to the bottom of your lungs, let your belly inflate. This may feel uncomfortable at first because it's so customary for our society to encourage 'sucking it in' but when breathing deeply we need to let it all flop out. Deep breathing will not only provide your vocal chords with the wind needed to make stronger noise, it will calm your nervous system and help release the tension you're holding in your belly. We want to release the tension in the upper belly (where the diaphrgam and Solar Plexus chakra are located) because we need energy to flow freely through this region of your body. Why? It's from this "gut" area that you cultivate your power and express it up and outwards into the world. It doesn't matter how beautiful your voice is, how much you love the music or even how deeply you're breathing; if you're Solar Plexus chakra is stuck in tightness you'll have an energy blockage and your music will have no power and no impact.
3. Meditate
Fear is just us distracted - instead of thinking of the task at hand (play music), your mind is wandering and worrying. Do I sound good? Do they like it? What if I mess up? What are the next lyrics? When you meditate, you're practicing controlling your attention to direct towards one thing. Each morning, you can do a breath meditation - keep bringing your attention back to the inhale & exhale. When you move through the forest or even through the city, you can do a walking meditation - keep bringing your attention back to the feeling of your feet on the ground or the smell of the air. Pick one thing and focus on it exclusively letting everything else blur into the background. Now, the better you get at this skill of focusing the mind, the easier it's going to become for you to apply it to music. As you start to sing or perform, the moment you notice your mind wandering, bring it back to the note, the sound of your voice, the meaning of the words (whatever you resonate with most). Just like a puppy who is learning to 'sit', the mind is always going to get distracted and it's your job to patiently train it to sit for longer and longer intervals. Eventually, you'll be so lost in the music, so present, you won't have any room in your head for those fearful thoughts. Remember, you're not afraid...you're distracted.