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"或"无法"。 face 在这里是动词，意思是"面对"或"直面"，指直面某个困难或令人不快的情况。 couldn't face it 表示"无法面对这件事"，指心理上难以承受或处理某个情况。