Sometimes you just have to let it run. My last 920 when factory resetting it took 4 hours to do the reset. (waitress at a restraint dropped it and broke the wireless charging) My new one I have not had to do any resets on .
that issue really happens when you installed so many apps, my suggestion is that you can try the hard reset rather than phone reset. How to hard reset: Nokia Lumia 920 hard reset