A positive Chinese idiom you can use to describe your situation of failing a math test is "失败乃成功之母" (shībài nǎi chénggōng zhī mǔ), which translates to "Failure is the mother of success." This idiom emphasizes that setbacks and failures are valuable opportunities for learning and growth. It suggests that through perseverance and reflection, one can ultimately achieve success by learning from their mistakes and improving their skills. It encourages a positive mindset and resilience in the face of challenges.