Depression.

Every-time she starts to feel the slightest bit of happiness, the depression hits and makes her feel worthlessness.
As she feels she's just getting her life back on track, that wave of sadness is enough to make her heart crack. 
She's loving life right now with her fiends, there's  nothing she enjoys more, but she can't help but feeling she's nothing but a bore. 
She hopes that one day she can overcome all this, but as there's no other feeling she knows how to feel other than sadness, the feeling of depression is the one feeling she won't be able to help but miss...