"Keep a level head" 或者 "Keep a cool head" 都可以表示把心态放平的意思。这两个短语的来源可能是源自于保持冷静的意思，因为在紧张或者困难的情况下，人们常常需要保持冷静的头脑才能做出正确的决策。

下面是一些英文例句和中文翻译：

"It's important to keep a level head during a crisis." (在危机中保持冷静是很重要的。)
"She always manages to keep a cool head, no matter how stressful the situation is." (无论情况有多紧张，她总是能保持冷静。)
"He was able to keep a level head and come up with a plan to solve the problem." (他能够保持冷静，想出一个解决问题的计划。)