She needed an heir for her kingdom, since their kingdoms were at odds and in the previous life, they went to war and it destroyed her that she was key to downfall to her nation. So by having a son with him, it pretty much serves as a method to force peace since he's to become Emperor and he has a bad case of feels for her and she has their son. So she is using his affection with their son and her leaving to keep the peace, since it gives him "hope" they can be together. After leaving, she left a letter revealing what happened in her last life and she doesn't really forgive him for forgetting they were sweethearts 10 years ago and he owed her. She pretty much rubs it all in his face what he did to her in the last life in that letter after she leaves to assumed the position of Queen in her home country.
He didn't want her to leave, wanted her to stay and be his empress and she's like, N O P E. I liked you, but you killed the one you loved you in my last life. I just wanted to use you for peace for my kingdom and get rid the woman who killed me. It's pretty clear she was sorta soft, more bitter sweet in her feelings towards him, but very much invested in the survival in herself and in her kingdom before him and their relationship. His heart and son are the way to keep her kingdom safe, so she's exploiting them to the max like a boss. I liked the ending.