Guild Wars 2

About to do some, just wondering if I fail, do I have to leave the instance and redo it all? Or can I just die and retry it in the same instance?

Thanks for any answers

For some you have to complete the instance with different decisions, for others you can do multiple ones at the same time. It depends on what you are working on, but for some death stops progression on the achievements and in that case, you will need to restart.

I recommend you looking up a dulfy guide or some such guide that better answers the by achievement questions.