turn the corner

turn the corner (pass/overcome the most difficult part of sth; the most difficult point of an illness)

have turned the corner (to have passed a critical point in a process) — миновать кризис; выйти из кризиса

Example 1: The patient has turned the corner. She should begin to show improvement now.

Example 2: The project has turned the corner. The rest should be easy. — ...критический/ напряженный период позади. Самое сложное позади.

see also
[break the back of sth]
[over the hill]
(be) over the worst