Sorry Brucie, no one can help. You are forever destined to have these words zooming around in your brain. You may be 95 with severe alzheimers sitting in the rest home, chattering away and infecting other ancients with ditties about "bears as thick as trees" until the nursing staff give serious consideration to converting the place to an insane asylum.
In the meantime, you will indeed forget lines and even entire verses of songs you have sung and enjoyed hundreds of times, usually about two seconds before you crucially needed to remember them, in the middle of the song and in front of an audience.
Welcome to the human race.