what did the doctor see?
Answers
There is no way we can know.
As far as I'm aware there is no clue in either that episode, another, or wider canon. A such without official word from the shows creators there's no way to answer definitively.
That's not to say we can't hypothesise!
It's my belief that he met himself (or possibly his 'War Doctor' regeneration). If you watch The Day of the Doctor and the events of the time war you'll see why he loathes that regeneration, he's both terrified and furious at it.
Furthermore there are some precidents for it, in the episode Amy's Choice they encounter the Dream Lord who is a manifestation of the Doctor's darker nature. He's a side of the doctor who 11 is terrified of and wants to overcome. In A Good Man goes to War River warns 11 about what he may turn into if he continues down this path.
11 has a darker theme, he has secrets which must never be spoken. He's running from his past, what he believes is his nature. As such I suspect he saw himself in The God Complex
Hope this help and please mark this as brainliest