Meaning: Never the most important person in a group (source); never fulfill ambitions (source).
Origins: 1924 Listerine ad (source). 
Example: I finally make president of the firm and they bring in a CEO - as usual, always a bridesmaid, never a bride.
Rewrites: always a groomsman, never a groom always a stable pony, never a thoroughbred always an assistant, never a department head always a supporting actor,  never a leading role always a seat warmer, never a celeb always a caterpillar, never a butterfly always a raindrop, never a snowflake always a nominee, never one who accepts the statue always a rivet, never a Bedazzler


Discussion: This ad tagline is rife with opportunities for rewrites and recasts. I would love to see what you can come up with.