Unless I missed it, no one's mentioned what I think your problem is.....you're describing a classic case of "stage fright". Some people with stage fright get "the shakes", sweats, start stammering, etc. Some simply forget their part. (music, lyrics, script lines, whatever) Having (fortunately) never experienced this myself, I don't have an easy solution. Do you perform in public regularly, or just sporadically? I suspect your issue is not practicing the song enough, (although that is important) but not playing before an audience enough. Maybe someone else knows a good cure for this? Good luck.