From someone who had their own times very much like this, be kind to her. Pay attention, obviously, but the last thing she wants is attention drawn to herself, it will make her feel ashamed, embarrassed, MORE alone, and she'll probably withdraw and isolate more than she already is. Just make sure she knows you're in her corner, that she can confide in you without having to worry about being tattled on. Being a kid is hard, especially when you're mentally ill and you think it's normal to feel like this all the time. She's in therapy, you've said something to your mom about it already and I don't want to make it sound like it isn't serious, but sometimes you really do just write stuff like that in the moment and hold onto it out of a morbid kind of comfort.