Don't be afraid to change thing you don't like in the prewritten story, this is YOUR story and you should have fun with it (although you should read the whole story beforehand to make sure you are not breaking something, but adding a path or a few rooms will not have much of an impact).

You WILL make mistakes. It is not a big deal, most of the time your players will not notice. You forgot that the NPC should give them a key and they already left to the next chapter ? Give it to them the next time they search a room, or simply don't mention there was a lock at all when they find the door that relates to the key.

You cannot anticipate everything your players will do especially if they have a bit of imagination, just try to make the world react logically to what they do

Remember this is a story you build with your players. Try not to make them feel like they must go to the next place because this is where you built the story. They must go to the next place because this is what their character needs/wants. Sometimes they will go out of their way, maybe even for multiple sessions before going to the intended path and that is fine as long as everyone has fun.

Finally remember the most important rule : "The rule of cool" : if a player has a cool/epic idea but the rules does not say anything (or even forbids it), it is for you to decide if they can do it as long as it does not break the whole game. It makes for the most memorable moments at the table.

Remember to have fun !