Christine has struggled with depression her whole life, though she resists labeling it as that. She began with dysthymia in high school which she didn't recognize as depression. She did everything she was supposed to, kept up with her homework and friends, obeyed her parents, didn't sneak out or drink or smoke weed, in part because her apathy prevented her motivation to act out.
This dysthymia was unnoticed by her family, because she kept up with everything she was supposed to. It wasn't something she ever talked about as a teenager, or that she could recognize or label. She did use writing as an escape from this low mood, escaping into her characters heads to bring herself some emotion. Her depression was worsened by Jamie's abuse and subsequent drug abuse and by her parent's preoccupation with Jamie which caused them to be less attentive to her.
Things improved in early adulthood, for a while, because of her new relationship with Jack. She was so thrilled, so enamored, that she was able to ignore his drug abuse and still keep a high mood. For the first time, someone was putting her first. However, after she gave birth, her depression worsened into suicidal ideation and disconnection from her child. She was committed to a hospital, where she was put on medication and for the first time was able to recognize and talk about her persistent low mood in high school.
Her mood improved with medication, but soon, the meds stopped working when she became more comfortable in her relationship with Jack, and thus less able to ignore his drug use. It was an unspoken secret between them, which made her unhappy. She felt she could not mention it to Jack, and that worsened her depression as the fuzzy feelings of a new relationship wore off.
Still, after her stint in the hospital, her family rallied around her to provide more support for Christine. Lily and Vivianne in particular reached out. This allowed Christine to finally admit that she'd been struggling with depression since she was in high school.
Her family apologized for missing the signs, while Christine struggles to let go of the resentment she feels for their focus on Jamie. This damages her relationship with Jamie for a while after he comes back. She understands why he got so much attention, but can't help but resent him for it. This resentment furthers her struggle with depression.
After she recovers from PPD, when her depression comes it hits her harder than when she was a teenager. Her depression comes in spells, and she moves from dysthymia to full blown depression, even struggling with suicidal thoughts.