Hi Ian, welcome to the forums!
I can relate very easily to your struggle with stage fright because it's been an issue for me my entire life. I was thinking about this today after I had to play a solo in front of everyone in one of my music classes, and something hit me about it all:
"We destroy arguments and every lofty opinion raised against the knowledge of God, and take every thought captive to obey Christ" ---2 Corinthians 10:5.
Performance anxiety is all from your mind, which the Word tells us we need to renew daily. Unfortunately in addition to freaking out while I play, I also tend to really beat up on myself afterwards because I'm a major perfectionist. Next time you think about your upcoming performance and you start to stress and worry, stop, and take that thought and turn it into something that God might say about you. Like, if you're thinking "Oh my gosh, I can't hit that note and I always sing the wrong pitch after this part of the song, I'm going to really screw it up," stop, and think "I may not be able to master those hard things on my own, but with God I can do anything because He gives me strength". And then, when you pray before you go onstage, have faith that God is going to help you out and is standing right next to you or is in the audience cheering you on. Here are some other scriptures that might help you.:
"I can do all things through Christ[
a] who strengthens me." ---Philippians 4:13
"For God has not given us a spirit of fear, but of power and of love and of a sound mind." ---2 Timothy 2:17
"I will lift up my eyes to the hills—From whence comes my help? My help
comes from the Lord, Who made heaven and earth. He will not allow your foot to be moved; He who keeps you will not slumber.Behold, He who keeps Israel Shall neither slumber nor sleep. The Lord
is your keeper; The Lord
is your shade at your right hand. The sun shall not strike you by day, Nor the moon by night.The Lord shall preserve you from all evil; He shall preserve your soul. The Lord shall preserve your going out and your coming in From this time forth, and even forevermore." ---Psalm 121
And here's some really good ones if you struggle with perfectionism:
"There is therefore now no condemnation for those who are in Christ Jesus." --Romans 8:1.
"But God, who is rich in mercy, for his great love with which he loved us." --Ephesians 2:4.
"Be anxious for nothing, but in everything by prayer and supplication, with thanksgiving, let your requests be made known to God; 7 and the peace of God, which surpasses all understanding, will guard your hearts and minds through Christ Jesus."
---Philippians 4:6-7.
*You could add verse 8 to this too.
I wish you well on your upcoming performance.