I understand and sympathise. Can you tell a story? If you can then you can memorize a song. Try it like this. Every story has a beginning a middle and an end. Think about the song like telling a story....first this happens then that etc. memorise the story. Now as you sing you will be telling this story and know exactly where you are in the story and therefore what verse comes next in the song..
Break the song into chunks to memorise. do not move to next chunk until you have learned the one before.
If you messs up the song during a performance do not worry. This happens to the best of us. Most of the time the audience do not even notice anyway. I have found that if I worry about new songs and remembering lyrics I usually forget them. Relax and go for the performance makes remembering easier.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055