Manga Like Chulsaengui Bimil

Hailen, the son of the Emmast Dukes. At the age of ten, he discovers he is not of the Dukes' bloodline. He even discovers that his biological mother switched him with the Dukes' son, and he subsequently hates the true Dukes' son, Gaillont. However, the more he conceals the truth and tries to eliminate Gaillont, the more he sinks into the abyss. Only on the verge of death does he acknowledge his ugly inferiority complex and close his eyes in regret. "Damn that inferiority complex, you foolish Hailen." When Hailen opens his eyes again, he finds himself back at the age of ten. Having regressed, Hailen abandons his hatred and obsession with Gaillont and resolves to one day abandon his position as the Dukes' son and begin a new life. He puts his plans into action, step by step... (Source: Ridibooks, translated)