This is a great question, Sharon, because it really gets at the heart of so many of our heroine's struggles. It underscores how important a parent's unconditional love is toward their children. When a child feels that they are not enough, and indeed, get that message through either verbal or nonverbal means, it's extremely damaging to their psyche.
Let me explain why.
Birth to age 7 is called the imprint period, and a child's brain soaks up everything like a sponge and a child will indiscriminately believe anything you tell them. The belief that she isn't supposed to be who she is will become part of her implicit memory.
Implicit memory differs from explicit memory in this way: explicit consists of factual data that you can consciously recall whereas implicit data consists of ingrained skills/thoughts that we know so well they slide into our subconscious mind.
Kate knows how to ride a bike, brush her teeth, breath...as well as believe that she isn't good enough as she is. She got into this habitual pattern of evaluating herself over and over again--and no doubt her mother reinforced that time and time again--and now that belief is part of her implicit thought.
So it would be no easy feat for her to overcome this ingrained lack of confidence without some interventions. Therapeutically, this would involve delving into her past, as there is probably one incident above all others that defines how she thinks her mother feels about her. Fictionally, her character arc will conclude when you revisit this incident somehow in her black moment.
As an adult, she has the mental capacity needed to challenge this belief about her identity that she adopted when she was very young. Other characters will no doubt be utilized to help build her self-esteem, as well. It will take patience and repetitive actions on their part (say, on the part of your hero) to help Kate believe she really is accepted the way she is.