Contrary to this experience, I had experienced my mother's coldness through the other episode.

coldness 是形容词 cold（寒冷的、冷淡的）加上名词后缀 -ness 构成的名词，表示"寒冷"或"冷淡"的状态。在这里指母亲表现出的"冷漠"态度。