I need help, i know its long, but im asking for someone, i need to know how to deal with my moms depression?

At around christmas eve my mom got really depressed and anxious, and couldnt sleep. Its been like this since. She just sits there and closes her eyes. She cant sleep at all. And i cant help her. Shes gone to many doctors and they say, her body is out of a certain chemical, and its so worn out (from stress and worries) that its having a really hard time producing that chemical. Shes on medicine, but its going to take a while to start having an effect, like maybe 2 or 3 weeks.I have only said a few words to her because she barely talks. I cant talk to my dad about it, because it wouldnt help, and my sister has been busy working and with her boyfriend. I cry myself to sleep at night, thinking about it, and her depression has really rubbed off on me. I love my mom more than anything, and i feel like ive lost her and i dont know when im going to get her back.and im so angry at God! because hes not helping at all. She spends hours a day reading the bible and writing stuff down and praying, and so has my family. i feel so abandoned and alone. I have my best friend to talk to. but all i want to do is cry, and i dont want to scare him. Im a 15 year old female. I find myself randomly crying hysterically, and then having to go somewhere to cry and let it out, because i dont want my mom to know how upset this is making me, it wouldnt help at all. I thought about calling my moms best friend, who is like my second mom, but i wouldnt be able to hold a conversation about this without an explosion of tears, and i just dont know what to do. i feel so abandones and lonely and helpless. And it hurts so bad, i just can describe it