I don't think she confessed strictly because of the promotion; I'd say she confessed because she could tell Stan was lying when she asked him directly whether he killed Vlad. She figures that since Stan is lying about Vlad, he's also lying about exfiltrating her, and he's also not really in love with her (since he's giving priority to his job, rather than her, by not revealing Vlad's killer), so she can't count on Stan to help her for personal reasons, either.
She therefore concludes that her work for the FBI will eventually end up getting her caught and killed by the KGB, since Stan won't help her escape, so she might as well come clean and try to get killed in the line of duty to the service of her country, rather die in shame as a double agent. Stan's threat to expose her illegal activities to her superiors no longer carries any weight, since pretty much all roads ahead of her lead to death, so she might as well take the one that lets her die with some integrity. It would be pretty hard for the rezident to fully trust her after this, so I expect that her mission for redemption will be her last. I also wouldn't be surprised if Stan learns of her betrayal shortly before her death, which might just send him off the deep end at the start of the second season.