This dream may symbolize your desire for acceptance and belonging in your waking life. The little girl represents a part of yourself that you may have been suppressing or ignoring. She keeps disappearing when you try to catch her, indicating that you have been avoiding facing this aspect of yourself. The fact that you are in a mental hospital in the dream could suggest that you are going through a period of self-reflection and introspection. The little girl finally coming out to you and getting comfortable with you could represent your growing acceptance and understanding of this hidden part of yourself. Taking a picture of her could symbolize your desire to capture and hold onto this newfound self-awareness. However, the fact that no one believes you when you try to share your experience could suggest that you feel misunderstood or invalidated in your waking life. This dream may be urging you to embrace all aspects of yourself and to seek validation and understanding from within rather than from others.