The character change both in Sid and Roshini post season 1 reflects psychological after affects. They've both build persona's that are opposites of their true character.
Sid way of coping was living with her memories in her heart while creating a casanova persona to keep girls away from him. He's not serious about them but the image is enough to keep any marriage proposal's falling through. Meaningless flings and business divert his mind and allow him to keep functioning. Again I'm pointing out Sid is emotionally mature. He has great emotional strength whereas Roshini doesn't.
Roshini's way of coping was to build a persona like her mothers. It keeps people away from people. If people can't come close to her they won't get hurt. Even if she has a relationship with Neil and his family there is still a distance; a barrier between them. She hasn't let them in completely. Her defense is her rigid personality that makes her unapproachable.
Also to be clear I'm not taking sides. I understand both of their pains because I can relate to both of them. I've hurt others but distancing myself and I've been hurt by others without being at fault. Roshini's logic is that Sid and their love is the root of all problems. They can't be together because they'll hurt each other and those around them. She thinks that being apart will hurt less than if they're together. She's wrong obviously but she doesn't see that. Out of two painful choices she chose the one she thought would cause less hurt in comparison for all of them.
Again if she's hurting Sid its not like she's having the time of her life either.
There are many times in life when we think we're doing the right thing when in reality we aren't. It isn't intentional but it still happens and it still hurts.
Kyunki Saas Bhi Kabhi Bahu Thi 2
