What song is this?

In one edition of Sixteen Candles (there's like 3 different versions with different music) there's a song thats not in the credits. As Molly and Jake meet in the coatroom at the dance this song starts with lyrics "How could you walk out the door on me, i thought that you were mine and mine for keeps..." I have searched endlessly and never found the song or artist. There's even a website dedicated to Sixten Candles that points out they dont know the song. Whats the song/artist?

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