Stupid question about difficulty

Since I beat game on normal it’s now asking for me to select difficulty before continuing solo. If I select normal again since I want to grind more gear, will the entire missions reset or do I still have my waypoints and such?

And If I select nightmare, can I go back to normal if it’s too much?

Your quest progress is saved in normal (by character) and you can return to it whenever you want etc to do whatever you want.

If you select nightmare you start at act 1 again on a higher difficulty. No worries, Act 1 isn’t harder than the later stages of A5 Normal.