When I first heard it sung by Judy Collins and later by Barbra Streisand, it seemed like it was about a woman speaking to a younger woman about how she never reached her potential and was upstaged by the other woman.

The actual story behind the Sondheim song is much different. Link . It is from the play, A Little Night Music, and is sung by a woman to a former lover, expressing her regret for rejecting his marriage proposal. Is that how you imagined the song?