This is In This Life, I Will Raise You Well, Your Majesty!.
When Ellisa opens her eyes and sees her young son, Leon, staring at her with eyes full of love and concern, she realizes that, by some miracle, she’s been given a second chance. In her past life, she thought making Leon emperor was the only way to ensure his safety. But if she had known Leon had never wanted the crown, or that laying a crown soaked with the blood of his enemies and beloved ones at his feet would mean he would take his own life before her eyes, she would’ve found another way. In this life, Ellisa is determined to find a way that will ensure both Leon’s survival and happiness. This time, things will be different.
A female wizard named Elisa is married by an emperor, who apparently wants her for her power. However, after bearing their first child together -- a son named Leon -- she loses her magic, and the emperor subsequently loses interest in her.
Elisa raises Leon very strictly because she wants him to be strong when he grows up, and in doing so, fails to show him the 'warmth of a mother.' She later regains her magic, but declines to use it to bring Leon's older step-brother -- the crown prince, Asensio -- back to life when he dies, which causes Leon to resent her.
Years later, through Elisa's machinations, the now-adult Leon eventually gains the throne, but isn't interested in becoming emperor, and instead commits suicide by drinking poison right in front of her. As he dies in her arms, she cries out, releasing a burst of magical energy.
She then opens her eyes and sees a much younger Leon, alive and well, sitting in front of her. Realising that she's now in the body of her younger self, Elisa resolves to change the future by prioritising Leon's happiness rather than his ascension to the throne.