Her father's death and the subsequent events turned her life topsy turvy. As she did not want to see him ruined by Ayesha's vendetta, she let go of him and closed his chapter the day she married Milsi. In Milsi she found true love, although this did not happen overnight. She realised what it is she was looking for and Milsi just fitted her right. Their opposite temperaments attracted them to each other and the similarity in their basic nature strengthened that attraction into a deep love.
In real life too, many childhood sweethearts dont remain sweethearts after they grow up, although a very few of them do. This is because people change as they grow up. Their outlook changes. The things they value changes. So a pair which may seem perfect when they are young may not appear so perfect as they grow and change. Prachi had changed. Neev has changed too. So Neev has to move on, either by trying to make his marriage work or by getting out of it and finding his happiness elsewhere. There is no use pining over the past love. It will only bring heartache.