Spoilers....
Everything was real, the dating sim part was all in his head. He did die in the first timeline, then got a second chance in life. In his second life, everything happened as exactly as the first, but him being hit a by car and dying happened much sooner than expected, the only difference is that he didn't die but lost his memories instead, then we saw how his parents regretted it and should've been more caring, they started to act like his biological parents. This is the start of the plot and where he met Sunwoo afterwards. As for why I'm sure it's a second timeline, his stepmother didn't get into a car accident like in the first timeline, so he searched up the incident on the internet with Sunwoo, proving it did happen and his stepmother is just fine, she didn't get hurt or got hospitalized. Another clue is that the ex who scammed him in the first timeline doesn't recognize him in the current timeline since he had Sunwoo now and a happy family so he wouldn't cling to others for love. There's no dating sim at all; it was all in his head created by his dying wish from the first timeline, it was probably a trauma response.
this one actually lost me. i’m certain the regression was real, but was he a.) actually stuck in a dating sim or b.) was it a trauma response? if option a, where in the timeline is the 2nd regression now that the game’s finished? if option b, woohyun needs to run from sunwoo ASAP?!? i LIKE fucked up romance but woohyuh’s acceptance/rationalization of his partner stalking, kidnapping, and assaulting him is beyond immersion breaking also if b someone PLEASE get woohyun antipsychotics