if u observe closely.. u can see various phases in Ishita's character.
At first, she was ready & happy to be married with Yash.
When the marriage got called off & Akash married her, she wanted Akash to love her back but he didn't, in fact he treated her like a piece of trash. Which brought in the revenge phase. She wanted Yash to be harmed because her life got spoiled. Though, she started to develop feelings for Yash because she felt he understood her, like one night when Yash sees Ishita & says, I can understand your feelings. She feels Yash understands her & she needs some one who can understand her.
Until G3's death, she wanted revenge on Yash-Aarti, even though she had feelings for Yash. After the leap, we have seen Yash-Aarti being much closer to each other, while still Akash didn't change but even manhandled her. So, now she started having the jealous phase.
She started believing Yash is her's & no one else can have him including Aarthi. The rest we know 😆