I had to quit my job the other day because my health has been so poor.
I had to resign from my job recently because of failing health.
どちらも「体調不良で先日仕事を辞めました。」という言い方です。
1) I had to quit my job the other day because my health has been so poor.
quit my job で「私の仕事を辞める」
「私はずっと体調不良です。」は my health has been so poor. と言えます。
2) I had to resign from my job recently because of failing health.
resign from my job で「仕事を辞職する」
ここでは「体調不良」を failing health としました。
ご参考になれば幸いです！
I quit my job a few days ago because I'm not doing so well health-wise.
I resigned from my job the other day due to a health condition.
体調不良 = failing health -> a (bad/poor) health condition / to not be doing so well health-wise
で = due to / because (of) / so
先日 = the other day / a few days ago / recently
会社を辞めました = I quit (my job) / I resigned (from my job) / I left the company / I left my job
アメリカで直接の自分のことについてなにか悪いことをあまり話しません。例えば：
A: How are you?
B: I'm bad. --> I'm not so good. (「I'm bad.」と言いません。）
「I'm not doing so well health-wise」と「I have a health condition」はもっと曖昧の言い方です。とても一般的によく使っています。
日本 
アメリカ合衆国 
