Under pressure, his determination weakened and he changed his decision.

weakened 是动词 weaken 的过去式。weaken 由形容词 weak（虚弱的）加上动词后缀 -en 构成，表示"使变弱"或"减弱"。 weakened 在这里表示"（决心）减弱、动摇"的意思。