Composure is a state of mind made possible by the structuring of one’s relation to one’s environment.

【句子分析】

made possible by the structuring of one’s relation to one’s environment 是过去分词短语作定语，修饰state of mind，它们之间不是“平行结构”，而是修饰关系。可以转换为定语从句：

Composure is a state of mind which is made possible by the structuring of one’s relation to one’s environment.

句子主干：Composure is a state of mind （沉着冷静是一种心理状态）

定语部分：made possible by the structuring of one’s relation to one’s environment. （人与环境关系的构建使其成为可能）

be made possible by 可以理解为：“之所以……是因为 ”。

【句子翻译】沉着冷静是建立在人与环境关系基础之上的一种心理状态。