Today's idiom is "go downhill ".
This means to slowly become worse.
Example 1
After his wife died, his health started to go downhill .
Example 2
I have no motivation to study hard. That's why my grades are going downhill .
Example 3
Last year I came to this restaurant and the service was great! But since that time, it has really gone downhill .
https://idioms.thefreedictionary.com/downhill