At the end of episode 104, Nurbanu makes a suicide attempt and is saved by Selim, whom she later learns is a prince.
In episode 105, her first scene is her learning that he was named governor of Saruhan. This province means nothing to her yet she is immediately on high alert.
She goes to question Canfeda about the subject, ignoring her quip about whether Nurbanu "had cut herself again".
She learns that Selim is now the best positioned to be the new sultan.
After that, her decision is made. Selim will be the new Sultan and she will be by his side.
Nurbanu is someone who believes deeply in fate. She wanted to die, and Selim saved her, and immediately after he was given the Manisa governorship. I think she started believing that these were all signs of fate. She was ripped away from her life in Venice to become a sultana like Hürrem. She believed so much in Selim because she loved him, but also because she became persuaded early on that their fate were linked, and their fate is the sultanate.
What's interesting about Nurbanu also is that believes in a "written" fate but she also believes in making your own fate. She arranges to go to the bathroom so she can see Selim. She bribes Sumbul (and unlike Hürrem she got the idea alone) so he will choose her for Selim's harem. She dares to ask Hürrem too.
Still, she consistently believes in and finds comfort in the guidance of the stars. It's no coincidence that it's not long after her attempt to get close to Selim in the hammam failed, despite her thinking that they were fated to be together, that she takes a risk to go outside and observe the stars.
When all other recourses have failed, Nurbanu is willing to forget her pride completely and beg and even asks for death, because she believes being Selim's wife and helping him become sultan (and this way gain power herself too!) is what she was destined for and she can't stand the idea of losing it, as it is her hope, her comfort, after the shock of her father's death, abduction and enslavement, which left her traumatized and suicidal.
I think Nurbanu was still suicidal at this point, but she found the strength to live in the idea of having a destiny awaiting her, a destiny where she would get both love and power, and that give her the sense of purpose she needed to keep going on.
