Sad, heart broken. Need help.

So, my daughter is 4 yo. And tomorrow I'm taking her to get a behavioral health assessment. I'm heartbroken, I never thought I would be here. She acts out no matter what I do, and recently has started to torment her cat, refuse to do anything she is told, she does things to intentionally hurt or annoy me and so, or the animals. She will throw something directly at me and then laugh when I tell her it hurts. She will refuse to eat, then go and sneak food into her room and horde it. I don't know what to do to stop her behaviors. I'm seeking out professional help, but I'm scared.., really scared. This is my baby, how can this be happening to us? I fear the worst. Since about 2 she has been going down hill, she has actually started things on fire. She has told me and so that she wishes we would die and she wants to go live with her grandma. Her grandma just moved to Florida 2 months ago, grandma gave her whatever she wanted and let her do whatever she wanted.. I don't know what I've done wrong. I love my daughter. I've tried so hard to help her. And I feel like I'm giving up, taking her to this place, having her talk to strangers, possibly be put inpatient, away from me for upto a week, with strangers. I'm so lost. I'm so hurt.