This quote comes from Nelson Mandela, who certainly earned the right to speak about perseverance and hard work. The seemingly impossible is something you can accomplish. You’ve done it before, and it’s important to remember that.
One of the stupid things you do at work is psyching yourself out. It’s easy to set up conditions for failure so it’s easy to take when you let it happen. Some tasks only see impossible because you build them up that way. Others seem impossible because they’re particularly hard. Either way, the point of a challenge is to conquer it and grow. You can’t do that if you concentrate on how difficult it seems.