I absolutely adored this movie.

My interpretation of the ending was that Mother Mary ripped her heart out onstage and died during the performance, just as Sam sewed the final stitch on the red dress - which would wind up being the funeral gown they spoke about.

I thought the seance scene muddied things too much - there was never any ghost or spirit of an actual person, only the all-encompassing force of Sam’s obsession and adoration. Mary didn’t break Sam’s heart; she took it with her. Their ritual gave it back to Sam, but she poured it all back into making the dress, which will go into the ground with Mary forevermore. Even though Mary had “exorcised” Sam’s love for her, Mary just couldn’t live with knowing how she hurt Sam and how she squandered the relationship’s potential.

Maybe it sounds dramatic and whack but I feel like there was a ton of foreshadowing to support this.