The Secret of The Doctors Name - *Spoiler*

There isn't one.

This entire three year arc boils down to a very simple premise - the baddie uses the Tardis to travel back into the Doctors past and rewrite history so that, this time around, the villain wins.

See previous post - Clara 

You were shown what the actual secret was in Saturdays Nightmare in Silver -  basically the Doctors true name is simply the order verification code required by a Tardis when an authorised pilot instigates a core overload - self-destruct. Similar to the security measure used on the planet-bomb in the previous episode.

The Doctor can't access it himself, the informations memory-locked under normal circumstances and can only be accessed by him under this exact circumstance via telepathic-circuit - it's a Galifreyian fail-safe measure. Hence his line to River in Forest of The Dead and the look of incredulous shock on his face:

The Doctor: River you know my name. You whispered my name in my ear. There's only one way I would ever tell anyone my name. There's only one time I could.  

So, why all the fuss?

In the original timeline - the one this has been created to overwrite - the Doctor blew up the Tardis rather than allow it to remain in enemy hands. The enemy uses the same machine in the moments it has left to travel back and rewrite time.

When one rewrites time one can only rewrite what originally happened by changing and re-purposing the original events to yield different outcomes - see  Knitting For Girls -  being as the whole point for the villain is to undo the Tardis's destruction it basically needs the Doctor to fail to be able to complete the Tardis self-destruct protocol and blow the ship up.

He has to fall silent when asked for the answer for the order verification code.

This was why the Silence were created, why everything else you've seen has happened: silence must fall when The Question that no living creature can fail to answer or answer false is asked.

To these ends the Doctors entire past has been rewritten - you'll have noted that, as Season 7 has progressed, there have been continual references to the Doctors past occurring. Everyone tends to put it down to it being simply just fan-nods to the upcoming 50th Anniversary the season finale leads into - narratively though its actually the Doctors own memories of himself being accessed, read, taken out of their original contexts and put back together again in an entirely different order.

The Doctors own History is being rewritten - he's literally being forged into being a different man.

This Doctor, the one that's been created in this rewritten-reality, never actually existed. A man who never lived never had a name. When the question is asked he has to answer true - and, from the perspective of the new History created for him, he will be.

The actual fact though is this whole deal is exactly the same as Donna after having been integrated into The Library Mainframe. The life she believed she had and was living was false.

To gain insight into the truth, in the original source story, Donna Noble required the perspective of someone dead who could see the world clearly for what it truly was. To those ends.

Welcome back, River Song...