Shutter Island is a much discussed film and it is a good time to be reading what people think about it. Screen Rant has a very accessible piece called Shutter Island Ending Explanation & Discussion and it is a good place to start in re-familiarising yourself with the film.
Some reading on Scorcese’s Shutter Island from Screen Rant. The website has a lengthy analysis of the Shutter Island ending and an explanation of why their analysis of the film fits with the story Scorsese intended to tell. Does their Shutter Island explanation match your theory? Find out!