The hardest time I had was during the month before I quit my previous work.
The toughest time was during the last month at my previous work.
ご質問ありがとうございます。
① "The hardest time I had was during the month before I quit my previous work."=「最もきつかったのは、前職を辞める前の一ヵ月間でした。」
② "The toughest time was during the last month at my previous work."=「最もつらかった時期は前職での最後の月でした。」
ご参考に。