I have difficulty remembering lyrics that don't reverberate in my soul. I sing in a group and have had to learn a few songs that have only marginally interested me. Learning those have been difficult. Once more repetition does the trick.

The real problem comes when you have the song hardwired into your brain. Then the danger is going into automatic pilot while singing it and then realizing you cannot remember which verse you just finished.

And there is never any guarantee that you will never forget the words. Once you are on stage, with the lights and the audience, it is very easy to blank on a song you have been singing for years. A former girlfriend once told me of seeing Aretha Franklin blank on a song in a live concert.