Does the main character have a mental Illness?
The main character might have a mental illness. She could have Schizoaffective Disorder. Some symptoms that this disorder includes are hallucinations, confusion, disorganized thoughts, changes in appetite, trouble sleeping, delusions. All of this symptoms I have listed are ones that our main character is experiencing. Our main character is experiencing 8 out of 15 symptoms for this Illness. Meaning that it is possible that she could have Schizoaffective disorder however I am no doctor this was an online diagnosis using healthline.org. I can not say for sure that she does have this mental disorder I am only saying that she has been experiencing some of these symptoms and she could have this disorder.
Is the God that she sees each night a hallucination?
The very definition of hallucination is an experience involving the apparent perception of something not present. So far we have no solid evidence that it is just her who can see "god" every night. However it is fair to assume that she is the only one who can see god in her room every night. If this is the case this is very clearly a hallucination.
Why does she see "god" every night
In my opinion god is her justice. She currently lives in a world where it is difficult to for a young girl to understand what is right and what is wrong. Which is way she created a character "god" to help her organize the information about right and wrong that her parents and other sources have been feeding her. This then allows her to understand her own version of justice. In other words she is having hallucinations to try and prevent her constant confusion about justice and what is right or wrong.