Heavier than Heaven.
Final Fantasy VII Ultimania Omega contains a story called Hoshi o Meguru Otome (Maiden Who Travels the Planet), which tells of Aerith's journey through the lifestream immediately following her death. It describes the world of being in the lifestream, and her place in the grand scheme of things despite not being alive. She encounters several deceased characters, including former AVALANCHE members. In chapter 5, she meets the familiar figure of Zack, who is, much to her surprise, in the lifesteam with her-- a sign that he, too, has passed on
Aerith previously assumed that Zack was indeed safe somewhere else, and due to his somewhat frivolous nature, she assumed that he had left her for good. She felt betrayed, and after almost four years of being faithful, she began to move on. Now, she learns the truth of Zack's whereabouts, and that in all honesty, he had never intended to abandon her at all. In fact, his last wish before he died was to go to Aerith in Midgar.
The reality of the situation hurts Aerith to the point of disbelief; she thought it wasn't possible that a soldier as strong as he was could have possibly be defeated. She begins to feel guilty about her assumptions, which turn to regret.
Nevertheless, Zack still jokes around in the afterlife, not having changed one bit. Although Aerith hasn't necessarily fallen completely out of love, his presence reminds her of the appreciation she has for him, and the reason why she would never forget him. He was, after all, her first love. However, because of her present feelings for Cloud, she decides that she can't simply pick up from where they left off. Although Aerith saw bits and pieces of Zack in Cloud (Zack was his previous mentor), she loved Cloud as a whole, with or without the characteristics of Zack. At the same time, she loved Zack for who he was as an individual, separate from Cloud.
Within the story, there's a lingering sense of tragedy; their relationship had fallen apart due to unforeseen and uncontrollable circumstances, and nothing more. Blame shouldn't be placed on what happened or what they did wrong, but perhaps in what they didn't do. So now, what of Zack of Aerith? Perhaps they make up and come to an agreement in the lifestream. But that's left for our imagination to decide.