Julie I am a fellow shaky forgetter of words which breaks my heart 'cos I love to sing but often don't!
Anyway, I was once lucky enough to get the chance to ask the singer Margaret Bennett for some advice. She told me to start with songs that have a good chorus where everyone can join in and that should give you confidence – songs like "Bonnie Bunch O'Thyme" and a few you have already listed.
Sorry I can't be of more help - I am still struggling with my own lack of confidence but I'm sure there will be plenty of suggestions forthcoming.
Tracy