I think you are already on the road to recovery - you went to join a choir and felt OK. I am assuming you felt panicky but didn't need emergency treatment? If that is the case and you managed your panic then you have already proved that you can survive going out and if you repeat this to yourself (and you love singing) then you can continue to push yourself forward (gently) just don't overdo it and congratulate yourself each time you come back from the choir - ie give yourself a treat - a whole bar of chocolate or a glass of something nice for example. Keep it up One day at a time springs to mind.
Things you find stressful that other people don't notice.