This question is opinion-based, which means that provided you give support for your response, you cannot be wrong. However, it also means that others may disagree; having said this, the following is my opinion.
Briony considered her story atonement for her actions, meaning that by telling it, and by altering the ending to something pleasant, she was able to correct the wrongs she had done. Unfortunately, it is highly unlikely that Briony felt redeemed. Due to the nature of their actual ending--with both characters dying and never having the opportunity to experience the relationship they wished to develop--it is more likely that Briony believed she had done the best she could under the circumstances, but only experienced deeper regret; in writing their love story, she gave herself a view of what they could have had, and the fact that she had been the one to prevent it is only more painfuul.