I guess she is not that angry, anymore, because she had some time to cope with the current situation. And as I mentioned elsewhere, Bihaan's attempt to bring her and Dhruv back together are actually laying the foundation for both, Dhruv and Thapki, to find final closure and move on from their past relationship - with confronting her with the past, e.g. Dhruv, Bihaan is unintentionally helping Thapki to let go of whatever anger she holds for Dhruv and with it, to let go of Dhruv, completely.
As for why she doesn't want to get back - there is this saying, that one may forgive but never forget. Even if Thapki has forgiven Dhruv for breaking his promise, does that necessarily indicate that she has forgotten that instance in her life? I don't think so. Once bitten, twice shy.
Another reason I'd think of is very simple: Dhruv is Bihaan's brother. Yes, morality probably does not play a huge factor in Thapki's rationalizing the situation, but the fact remains that Dhruv and Bihaan are brothers. If Thapki were to marry Dhruv after her divorce, she still would be confronted with Bihaan on a daily basis, living under the same roof as him, her past and the betrayal would always be in front of her. She wouldn't be able to move on - vice versa, e.g. if she remained with Bihaan, she'd move on completely from Dhruv. Why? Just remember the episode of her breakdown after finding out whom she got married to- more than her marriage breaking, what was she devastated about? The fact that Bihaan broke the marriage due to her stammering. Dhruv cannot heal that wound, only Bihaan can (whether as a friend or a lover).
And the love factor is also important. Dhruv has always loved her more, there's no denying of that. Would she love him equally as much, she would never let go - but she is doing exactly that. It looks like Dhruv was a chapter in her life which she is closing now as she is over with it, no hard feelings but not a big enough attachment to cling to- She tried, it didn't work out, let's move on. Maybe she just accepts that her and Dhruv's relationship was only this long-lived and life has different plans for both of them, now.
Udne Ki Aasha
Jaane Anjaane Hum Mile
Fan Fictions
