Hmmm, i had the feeling that shagun is clearly thinking about getting back with raman. Not saying that it will happen tho.
She said she was the culprit instead of adi so that raman felt protective towards her (be if for adi's sake), she is trying to get to spend time with him and to have him protecting her against the world and so far it's working.
If she had said the truth : "adi did the accident", raman would have bashed her and told her than she can't keep custody of the kid since she doesn't know how to educate him.
Since ishita came into raman's life, all shagun wanted was to make sure that raman still have feelings for her and she wanted him to respect her while insulting ishita. Because in her mind being the first wife/first love and the mother of the children gives her that right. She doesn't realize that respect is something that you earn, not something that is given to you.
She left raman because he had devoted love but no money, and went to ashok who had lusty love and money. And now she sees that raman (who loved her like she was a goddess) has money and she THINKS he will love her like he used to do (+ give her as much money as she wants) so she want to go back with him.
Also it'll allow her to insult ishita if raman leaves ishita for her (which won't happen)
She doesn't love him nor she has love to give, all she wants is to be loved and spoiled.