You must be mentally prepared.

你要有思想准备。

youdao

But what I have found helpful is for me to be mentally prepared to face rejection.

但是我发现对我有帮助的是在心理上做好面对拒绝的准备。

youdao

But we have to be mentally prepared for the problems with which we may be confronted.

但是我们也必须对可能会面临的问题(problem)有所思想准备。

youdao

To be mentally prepared for such extreme and demanding environments is incredibly difficult.

BBC: Sir Ranulph Fiennes: Coping with extreme weather

"I want you to be mentally prepared in the event you are not retained, " the sergeant major wrote to Sgt.

WSJ: When the Fighting Stops: Troops Coming Home from Afghanistan

Passengers should obey the directions of the flight crew, but they should be prepared, mentally and physically, to act.

FORBES: A How-to, When-to Guide For Airplane Vigilantes