A Mistaken Marriage Match: Mysteries in the Imperial Harem - Chapter 74

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!

- Description from Novelupdates

Having a rough life growing up, Wang Tong had numerous dreams of living a better life. However, his alcoholic foster father sells him off as a space settler of the distant planet Norton for some pocket change. Soon after his arrival, the Zergs overran the planet and obliterated its defenses in a matter of minutes. What awaited Tong on Norton was more than just an unbearable environment – its gravity alone was five-times stronger than earth – there were also swarms of bugs, constant hunger, thirst, and desolation.

While he lives a year of total carnage and survival, humanity’s Confederation eventually overcomes the Zergs on Norton and the powerful survivor Wang Tong is given a recommendation to a once famous, but now dying military academy where he begins his life anew.

- Description from Novelupdates

The protagonist, having degenerated into a “loser” after being unable to find employment, gets caught up in someone else’s attempt to commit suicide-by-train and ends up traveling to another world. The numerous cheat powers he demands from God are all granted to him. But contrary to expectations, these powers bring about disastrous consequences for the protagonist.

Having reincarnated into another world, the optimistic protagonist (Yuuri) starts his(her) heart-warming training days with his(her) teacher.