Doctor Who: Why can The Doctor suddenly travel back on his own timeline?

Prior to the Matt Smith seasons of Doctor Who, The Doctor had never been able to travel back on his own timeline (or that’s how I took it). I never quite understood that point to be honest, but just accepted it as a way around the inevitable question of “why not just going back to 5 minutes before sh*t hit the fan and stop it before it even began?”. Now, in episodes like “The Pandorica Opens” and “Big Bang” he’s jumping back and forth all over the place, even seeing and talking to himself… Someone please explain to me how this isn’t breaking the rules of time travel as it’s portrayed in the show, and if he can do these things, how/why he can’t go back and stop things ahead of time as I mentioned above.