"Mentally Ill" is a list of contradictions, and that is the point. The speaker is happy and depressed, silent and screaming, afraid of living yet tired of dying. Nearly every line sets two feelings side by side and refuses to pick one. The effect is a mind that feels trapped, pulled in every direction at once, with no way to settle.
The speaker also hides this behind a smile and struggles to speak up, so the poem becomes the "cry for help" it names. Yet they ask to solve their own problems, which shows how torn they are about being seen. The ending is the most painful part. Pain is both the thing they want gone and something they feel attached to, even love. By the last line they need pain to get rid of pain. The mood is exhausted, confused, and raw, and the unpolished writing makes it feel more honest, like a private confession.