Major/Commander/Sub-Commander Sela is disgraced after several failed missions thwarted by Picard. And so she has a need to rebuild her own personal power-base along witha personal grudge against Picard and the Enterprise.
Her standing is further reduced by the Alliance to fight the Dominion, and with a looming friendship with the Federation, her hardline standing is seen as a flaw...
Thats when she discovers Picards clone, and begins to plot events. Ensuring some covert help to the Remans, and helping Shinzon to grow and become a Reman trooper, starting his rise to power... For this she was relying on several of Picards personality traits, her covert help and the Reman Viceroy. And in the meantime she enabled some secret construction facilities (ie; Tal Shiar, but a deniable recource) to begin construction of the Scimitar...
Once everything was in place, the Scimitar, Shinzons placement as leader of the Remans, she began part B to her plan, and plotted and carried out the assassination of the High Council (having sidelined her career, it became a pleasant task)...
And so the events of the film began to unfold, until Shinzons instability threw the first couple of wrinkles into the plan. THis was when the little known plan C came into effect. As Sela had planned to ensure that her powerbase would be rebuilt no matter what...
And so she tipped off Donatra and brought her Romulan associates into a position of Neutrality. leaving Donatra to attack Shinzon and either win and gain the trust of the Federation for all, or loose and be destroyed (thus destroying proof of Sela's involvement)...
Now the film closes, and Sela steps up, and claims credit for some of her actions, while disposing of some assets that could embarrase her later (the unfortunate accident involving one particular polititian/assassin is a little convenient, but little is proved, and many Romulans think it fair justice)...
So now Sela sits as one of the senior ranking Tal Shiar officials, and as a hero of the Empire after having saved it from the mad Shinzon, and fakes a veneer of neutrality towards the Federation, while working on increasing her political power and looking for new ways to get revenge against Picard...