Lena's mother is frequently presented as an example of what not to be: happy, playful, caring, loving. Basically having any sort of emotion whatsoever is a Very Bad Thing in Lena's world. As a result, Lena's feelings about her mother alternate between betrayal—Mom killed herself and abandoned Lena—and fear—Lena's going to end up like her mom, oh no.
Despite these negative feelings, Lena's already fragile emotional state is shattered when she finds out that not only is her mother still alive, but she escaped from a mental institution where she was being held for life. So she's out there, somewhere. Still being her mom and all.
For some strange reason we can't really understand, Lena feels even more betrayed by her mom's escape than she does by her being a societal rebel. She pictures her mom "orbiting as far from me as a distant constellation" (23.13). Quick astronomy lesson: Constellations don't orbit.
Ah, we get it know. This analogy illustrates how Lena thinks the universe revolves around her. So she resents her mom's escape because it means that Lena couldn't find her, even though it's like, obviously the better thing for her mom more generally.
In Delirium's dystopia, Lena's mom's mood swings are written off as symptoms of amor deliria nervosa. But in our world, she would be diagnosed as bipolar. She's "always laughing" (20.59) and staying up all hours of the night, except when she cries and cries and "there were days when she didn't get out of bed at all" (20.62).
Looking at her illness in this light makes her treatment all the more appalling. She's locked in an asylum straight out of the dark days of mental health's past, and given no hope of treatment. This "cure" seems to be no better than a lobotomy, and the doctors make no other attempt to treat her.
Nothing about mental illness is ever simple or easy. Lena's mom carves the word "love" into the wall and escapes through the hole. Sounds a little bit like a Bugs Bunny prank, but whatever works.
In the end, Lena's mom lives by the words she tells her daughter: "I love you. Remember. They cannot take it" (4.34). They couldn't take her love away from her, for better or for worse. The very same thing that got her into trouble, gets her out of that darn asylum.