I'm pretty sure that this is a yulti fic.

Yuri has face blindness where she can't remember anyone's faces. Her ex, I think it was Jessica, left her because she doubted her love. I mean, come on, if yuri trully loves her, Yuri should at least be able to recognize her face amongst thousands right?? In the end, Jessica realizes that she couldn't move on, she was still so much in love with Yuri. Jessica returns to Yuri's side, only to find that Yuri does not recognize her and doesnt seem to be effected by her leave, thus she could only watch Yuri from affar. 

Yuri is now dating Tiffany and the same thing happens. At first Tiffany was understanding of Yuri's condition, but she too, begins to doubt Yuris love for her. Tiffany ends up leaving Yuri and runs into Jessica where the latters tells her of her regrets. Tiffany returns to Yuri's side, watching over her as she works in Sunny's shop. A guy tries to hit on Tiffany, but Yuri intervenes, stating that Tiffany was her girlfriend. For some reason, despite Yuri's condition, she can still recognize Tiffany.

This was a one-shot. I seriously reccommend you people to look for it and read it. It's an awesome read.

UPDATE 12/5/2016 : I was able to find it. The fic is called Faces by crushergo.