YOU SAID:

I started to feel good, until I didn't.

INTO JAPANESE

私は気持ちが良くなるまで、気分が良くなり始めました。

BACK INTO ENGLISH

I started to feel better until I felt better.

INTO JAPANESE

気分が良くなるまで気分が良くなり始めました。

BACK INTO ENGLISH

I started to feel better until I felt better.