This fledgling field has already proved the truth of what once passed as bromide: What doesn't kill you can actually make you stronger.

【分析】这个新兴的领域已经证明了曾经被认为是陈词滥调的真理：那些没能扼杀你的磨难实际上可以让你变得更加强大。

【分析】

This fledgling field （主语）has already proved （谓语动词）the truth （宾语）of what once passed as bromide（宾语的定语）： What doesn't kill you （主语）can actually make（谓语动词） you stronger（复合宾语）. （整个句子是 truth 的同位语）