Very normal what Federer is experiencing. He has been on a natural decline since 2006 anyways. I mean fact is you can't keep it up.
To say he has nothing left is premature though. I mean if you take a look at what he has achieved since Wimbledon last year, there is absolutely no reason he can't win a few more slams. He made the semis at the US Open and Australian Open, and won the second most important event on the main tour, so he still has some gas left in the tank.
I mean if you take a look at his matches where he has lost since Wimbledon, I think he has just made some really poor tactical decisions rather than just being outplayed/fatigued/etc, especially against Novak in Australia. Yes, Novak was playing extremely well, but I thought Federer had figured out the correct tactic in that second set, as did the commentators. Yet when he had the lead, he reverted back to how he played Novak in the first set and let Djokovic just counter-punch, feed off Federer's pace, and allow Federer to make a lot of poor errors.
Sampras was in this similar situation after he won his last Wimbledon in 2000. In the biggest matches, he just didn't play the right shot at the right time, but he obviously struck gold at the US Open.
In short, it is just a mental thing and it is totally understandable. He is close to retiring, so he has that pressure in that back of his mind that he doesn't want any of these matches to slip away. The positive is that his forehand is still arguably the most lethal on the tour, his serve is the most accurate of the top players and he can still match with anyone physically.