@MoodyUsername I had to go through my entire reading list to find it, but I finally did.
Maou-sama Retry, then its sequel Maou-sama Retry R. The mc is sent in another world in the body of a character in a game he created, and there's something wrong with it. His personality is different, but he somehow always ends up acting more like the character than himself, without noticing.
There's also one that's based on the mc being a chuuni and lying, except the lie was the truth and so the mc is the only one who doesn't believe in what he said. It's called "the eminence in shadow". Pretty funny. Tho he's not technically forced, as this one wants to be badass.
Also, there's the good old overlord, that's literally what happens to momonga, being badass against his will.