@Shaavi, Payal, and Geena: Great discussions about the motivations for Sona's choices during marriage. I agree with all of you, because I think Sona was operating in complex ways, and had multivalent reasons for all her actions.
Where the perfect bahu pageant was concerned, it was definitely a reaction to Ishwari's insistence that she couldn't be a traditionally good bahu, and motivated primarily by her pride. She wanted to prove that she could excel at this too, at the same time focused on how important Ishwari's approval was for her relationship with Dev. That whole phase, I think she was quite self-absorbed, but she had a moment of realisation after the double dinner, when Dev said he couldn't communicate his relationship with Ishwari so easily, and went back to being herself with Dev.
This was apparent when she planned the resort night for both of them, and they proceeded to plan their honeymoon. She decided to stop making the situation ideal with the family before she allowed herself to enjoy her love. Just when Sona was beginning to shift her main focus to Dev (what he also wanted), Ishwari sabotaged them with her concerns about Neha. This time, it was Dev's turn to put his family and his personal emotions before his relationship with Sona, and just like he had tried to support her during the perfect bahu phase, she tried to support him in this.
I think that was the primary motivation for her actions vis a vis Neha's situation. She saw how upset it made Dev (who can forget that scene where he broke down after canceling their honeymoon?), believed that he was dealing with it in the wrong way, and thought she had the right solution to the problem. She wanted to save Dev from himself and from his family so that they could have the relationship she dreamed of. Here is where I agree with Payal and Geena. Her intentions were selfless but she failed to understand why Neha's needs weren't the same as her own, and why love wasn't the be-all, end-all answer to her problems.
And while her pride wasn't the main motivation here, it did play a minor role when Dev once again berated Ranveer's financial situation and his inability to care for Neha. I think Sona felt responsible for changing Dev's mind about the relationship between financial status and love because she took his words personally. Like we discussed a few days ago, though, Dev was never making an universal statement about it -- he was talking only about Neha's particular situation. Sona's intentions here were selfless, but her execution was ill-informed and impulsive. I think her assumption that her view of the situation was right, and Dev, Neha, and Ishwari's wrong, played heavily into this.
I guess what I am trying to say is that I agree that Sona's fatal flaw was her pride, stemming from the confidence that her black and white view of the world was the right one. BUT I think it played out very differently in different situations, and was not the only quality that defined her behaviour during their marriage. She did a lot of things right, and was very generous with Dev on a lot of occasions. But when push came to shove, it was the hit to her pride that dealt the fatal blow from her side.