Lyric Analysis:
I hate it here so I will go to secret gardens in my mind
People need a key to get to, the only one is mine (I Hate It Here)
This line references The Secret Garden, a children's novel by Frances Hodgson Burnett in which a young girl finds the key to a locked "secret garden" that she begins to tend and care for. Like that garden, the secret garden that Taylor cultivates in her mind requires a key to access--that key, presumably, is her music.
In the novel, the secret garden serves as a symbol for rebirth. When spring rolls around and the garden, now tended to by the protagonist Mary Lennox, blooms again, it is coincided by the restoration of Mary's friend, Colin's, health. After years of being kept locked away in a small room, Mary begins taking him for strolls through the garden and, with exercise, he begins to recover from the worst of his ill health.
Similarly, the secret gardens in Taylor's mind correlate to her own rebirth. Throughout the verses and in the first half of the bridge, Taylor sings of how her world has torn her down, stating, "This place made me feel worthless," but the second half of the bridge is dedicated to how the secret gardens and lunar valleys she imagines help her cope with it. Just as living in the dark room he had been confined to for years tore Colin down and wandering through the garden subsequently helps him treat his illness, leaving her reality in favor of tending to her daydreams renews Taylor's ability to cope with the daily struggles of life. She hates it here, but that's okay--because she has her secret gardens in which to heal herself.