Two things have helped me.

Some years ago I started practising (voice and guitar or voice and lute or one of those three alone) with my eyes shut, as a way of focussing on the performance piece. I found it really helped. Then, when in public, I closed my eyes and found myself in the same focussed mental space as when practising, so the focus is on the song or tune, not on the audience.

One day, many years ago, when waiting nervously backstage at a festival, I suddenly realised that my nerves were simply adrenaline, the same thing that gives the energy for a good performance. So I just decided to focus that energy positively on performing well, instead of negatively on wondering what could go wrong.

Hope this helps.