I knew someone had to tell her but I couldn't face it.

couldn't 是情态动词 can 的否定过去式，表示"不能、无法"。 face 在这里是动词，意思是"面对、直面"，指面对困难或不愉快的事情。 couldn't face it 表示"无法面对这件事"，强调心理上难以承受或处理某种情况。