It was only in his letters that he truly opened his heart and shared his deepest fears.

opened 是动词 open 的过去式，open 本意是"打开"，这里比喻为"敞开心扉"。 his 是形容词性物主代词，指"他的"。 heart 本义是"心脏"，在英语中常用来比喻"内心、情感"。 opened his heart 是一个固定表达，字面意思是"打开他的心"，实际表示"敞开心扉、吐露心声"的意思。