I hadn't been to school for a long time until yesterday.
I hadn't been on a diet for quite a while, but I started one again yesterday.
最近していなかったけど、昨日したのなら、過去完了形を使って表現すると良いでしょう。
ーI hadn't been to school for a long time until yesterday.
「昨日までしばらく学校に行っていなかった」
ーI hadn't been on a diet for quite a while, but I started one again yesterday.
「しばらくダイエットをしていなかったが、昨日また始めた」
ご参考まで!