He finally repented his mistakes and decided to turn over a new leaf.

repented 是动词 repent 的过去式。repent 由前缀 re-（表示"重新"）和拉丁词根 pent（来自 paenitere，意为"后悔"）组成，意思是"对自己的过错感到后悔并决心改正"。repented 在这里表示"已经悔悟"的状态。