You people !
Can't you see, everyone around Fiona is winning or at least on a path of getting their **** together besides FIONA.
This totally changes the dynamic of her character, where she fits in with the rest of the family and what the means for her next season.
We knew her and Sean wouldn't make it past the alter, and him getting shot by Frank or overdosing on drugs moments before the wedding would've been forced.
Frank exposing Sean for what he truly is is what brought it all together.
Sean breaking up with Fiona makes a lot of sense considering he had been using the whole time he said he wasn't and he knew that if he had tried to work it out with Fiona, she wouldn't look at him the same way; also, he was making excuses for his drug use....yeah, it was over.
Now if only the family could move on from that asshole father of theirs !