Don't worry about missing stuff, you'll probably end up going into NG+ regardless. Some of the bosses and all of the DLC content is designed to be tackled after beating the game and since there's no real way to tell it apart from the easier content, if something mauls your face off, you might want to leave it alone for the time being. You can beat everything on your first run if you really want to, but you'll need to grind a lot or figure out how to cheese everything.
The world is wide, but you don't necessarily have to explore it all at once.
Or maybe you enjoy fighting stuff way earlier than intended. The game can be beaten at level 1 with no colored souls, if you find all the tools and know where to use them.
There's no wrong way to play the game, except for reading spoilers. Don't do that!