I regret trying to explore so much because I'm bored before I've even got to act 3.
I think I'll leave it until next year and then do a new playthrough ignoring most the side stuff. Or just watch a video of the ending but that seems a bit of a waste as I've done so much.
There's no definitive ending video you can watch as every playthrough is different and choices you make throughout all three acts will affect your eventual ending. Don't forget you can respec your character (and companions) to any class and build you like, which can help spice things up if your combat and encounters are getting a little stale.