My frustration was deepened by the fact that the day before I had written in the diary some libellous remarks concerning my parents.

deepened 是动词 deepen 的过去式形式。deepen 由形容词 deep（深的）加上动词后缀 -en 构成，表示"使变深"或"加深"。在这里指"沮丧感变得更强烈"。