Why did I fail?
My instructor signed the endorsements, why did I fail then?
Because you weren't mentally prepared for a checkride.
That your instructor "signs you off" doesn't necessarily mean a guaranteed pass. The instructor signs a student on the basis of knowledge and skills, not on the mindset of the student the day of the checkride.
Unfortunately your instructor can not be in the checkride room that day, giving you a look when you doubt an answer or when you are not sure what to say. That day you are alone with your knowledge and everything you learn during training, that day is the moment you have to show a DPE you are worthy of a certificate or rating.
The day of your checkride you have to come well rested, you don't have to be awake all night to review stuff, you already know all the stuff at that point, so a good 8 hour sleep can save you the next day. Have a nice breakfast, that would give your mind all the power needed to overcome the tasks on your checkride.
And more important, RELAX, you have been studying, you know how to do the maneuvers, you know the procedures. If your CFI signs you, that means you are ready, the DPE is just there to make sure that you satisfy the FAA requirements on the ACS.
Javier Fernandez
Chief Flight Instructor