你只知三年痛却不知七年痒
[nĭ zhĭ zhī sān nián tòng què bù zhī qī nián yăng]
In English, it's 'You only know three-year pain but do not know seven-year itch'. This idiom is used to comment on people who think they understand something/someone through a period but fail to realize a deeper, longer understanding over time is still needed.