Man all the endings for Ocarina of Time are pretty depressing now that I think about it.
Adult timeline: definition of pyrrhic victory as Ganondorf soon returns and Hyrule is permanently flooded.
Child timeline: best timeline in terms of long-term outcome, but the hero of time is completely forgotten.
Downfall: Not only did the HoT fail and fucking die, but plan B results in the corruption of the sacred realm and Ganon's permanent threat to Hyrule.
All Zelda had to do was keep or smash the Ocarina of Time, and Ganondorf would not have reached the Sacred Realm and claim the Triforce, he may have been able to smash through the Door of Time, but he would never be able to pull the Master Sword to enter it.
I think that's why Zelda feels the way she does in the end, and blames herself for all this.