If I have a emotional problem with a song I make sure I have sung it so many times beforehand that you can sing 'through'. i.e. almost in a mechanical way, it's just another song for the very short while it takes. You no longer have to worry about the words, you are able to blank your mind off for the short while it takes to get through it... and then come the tears in huge waves. One of the most courageous singers I have seen recently was a friend when she sang at the funeral of her husband who died unexpectedly and at a young age. Again she sang 'through' the song and there was not a tear until afterwards - everyone else was in tears though! Good Luck