It's funny that you should say "this reminds me so much of Shutter Island, because that is *exactly* the movie I was referring to when I was first vaguely talking to you about the movie, and I told you that there were some movies that I could compare it to, but I can't even make the comparison without it being a spoiler.
If I had told you that this movie is like Shutter Island, that would have clued you into the fact that there's a definite psychological twist near the end...but I'm really glad you made that connection yourself once you saw it.
This is definitely one of those movies where the ending kind of *makes* the movie, and is the reason that this movie is considered among the upper tier of psychological horror. Without the amazing reveal at the end, this movie would just be kind of a 'meh' mystery horror movie - nothing particularly remarkable about it.
But like Shutter Island, once you know the big reveal and you know what's actually happening, it kind of casts everything that came before it in a new context, and for me personally, makes it much more interesting to watch.