Hi,
1. After bitter and unstable days and months, he was determined to not get discouraged and and his persevering efforts was repaid. Finally he became a famous actor.
- Is this natural to say?
'Was' is wrong, because 'efforts' is a plural word.
I'd also make a few improvements in style and vocabulary.
Efforts don't idiomatically get 'repaid'. Instead, efforts 'pay off', (ie pay you a result.)
1. During bitter and uncertain days and months, he was determined not to get discouraged, and his efforts and perseverance paid off. Finally, he became a famous actor.
Best wishes, Clive