My reading was that Boimler’s slip-up and the knowledge that she wouldn’t be a significant part of Spock’s life (at least viewed from a historical perspective) was what caused Chapel to pull away from Spock, and end up sabotaging the relationship. But tragically - time-travel shenanigans and all that - who’s to say whether or not that’s the way things were always going to happen?
The opportunity the fellowship provides allows her to envision a positive, worthwhile future for herself, where she is free from the boundaries she’d previously imagined, and can let go of her disappointment that the path she yearned to travel with Spock was one she wasn’t destined for.