面上无比镇定心里兵荒马乱
[miàn shàng wú bĭ zhèn dìng xīn lĭ bīng huāng mă luàn]
This translates to 'Seemingly Calm on the Surface but Panicked Inside'. The metaphor '心里兵荒马乱' portrays one's internal turmoil similar to a chaotic battlefield. It vividly depicts a person who outwardly looks composed yet experiences great stress and anxiety within.