Where do i start?

Yesterday was awful!! It started out decent. I took Angeline to counseling and it was just us, having our mommy n me time. She seemed fine,we had chocolate milk n a donut on the way and she done her session and we came home.. We had been home an hour and Halie brought me a big piece of plastic, she told me that angeline had been playing with it,I asked how and she said she was inhaliing it. Then the following just literally rocked my world. She said that Angeline has been telling her and Toni that she wants to die,and she remembered when she had held the bag over Kaylees face and the talk we had, that she remembered I told her that plastic can kill you if you attempt to  swallow it, and she didnt care. She then took the plastic wrap and held it over her face and began to inhale, Halie grabbed the plastic wrap away from her face and thats when she came to me. I called Toni in the room and asked her what Angeline said, and Toni pretty much said the same thing. I then call Angeline in the room and she flat out lies to me. I asked her again and she denies it. I tell her I heard otherwise & it was EXTREMEMLY important to tell me the truth, I ask her if  she was lying to me, she then fessed up..At that point Dave gets home and he figures out whats going on and we try to get ahold of her psych.which we cant, its busy every time we call. we decide to take her in to the er.. we began to get the  babies ready, i try one more time,and the office manager answers. I explain whats going on and she tells us tobring her in immediately. We get her there n after a brief examination and other things, we find out her antidepressant is no longer working for her. He also suggested we up the adderall and we also put her on lithium. I ask why the lithium,he said it was a well known drug proven to stop the suicidal tendancies n thoughts, it was also a mood stabilizer. He did say more about it but at that point I was more shaken up... we go ahead with the chang of meds and he dismisses her to go home with us until tuesday. We have to bring her in for another eval and if no change we will proceed to other "options".
So far she appears to be okay. The side effects have hit her and HARD. Poor baby has awful stomach cramps, and began to vomit,she is also sleepy..
I feel terrible,beyond terrible! How could I have missed all the signs? I failed my daughter as a mother. I feel as if I was paying to much attention to the other kids and ignored Angeline. I know very well that this was not the case, I spend  the same amount of individual attention with all the kids and we do the family time..No one is neglected, so to say. But still my baby is sick and almost was successful in committing suicide.  I thank The Lord that Halie was quick and was able to react the way she did and told me. But Why oh why did Angeline not come to me? I just dont understand! I want to help my baby, but I feel completelly uttterly helpless. I failed  my daughter as a mother. I just dont know where to turn. I know I done the right thing but what do I do now?