I interpreted that song very differently. When Boimler spoke with Chapel, she didn’t just realize that her and Spock wouldn’t be together long term but also realized that Boimler didn’t really know her like he knew Spock.
Spock goes on to do amazing things and every detail of his life is recorded in books that people over a century later will read and, essentially, worship him. Chapel isn’t even a cliff note. In her mind, she must feel like she makes no difference and gets down on herself. When she gets the fellowship, it renews her confidence and let’s her know that there is a whole universe of possibilities in front of her.
That was my interpretation of her feelings in the song but I can see others as reading it differently.