Perhaps I missed some episodes, but I can recall that in the time that Abhishek was trying to win over Mihika (with help of Mihir), she was trying to get close to Mihir again (with help of Abhishek).
Then apparently I missed a bunch of episodes...
Ruhi, Ishita and Nidhi were dead.. Oh but not really..
Mihika and Romi are a couple. Sarika is evil and vanished.
In the case of MiMi it could very well be that they did not fall out of love but love evolved to the next level.