I should have been friendlier and more interested, rather than quiet and withdrawn.

withdrawn 是动词 withdraw 的过去分词形式，在这里作形容词使用。withdraw 由前缀 with-（表示"向后"）和 draw（"拉"）组成，本义是"撤回、收回"。withdrawn 形容人时表示"内向的、孤僻的"，指性格上倾向于回避社交互动。