People! I need your help! I'm so confused what is happening... can someone explain, please...

From what I understand :
Both of them went back in time but only Iheon having original memory while Huimin has split personality due to PTSD of his past lives. And from what I understand, I think Iheon going back in time more than once because he use word "always".

While Huimin thought he is in the novel, real Huimin is actually also his personality not just novel character.

I don't read the novel until finish so idk, it just my understanding so far.

They both went back in time, iheon died first, huimin sees him dead and his mind breaks, and when he kills himself or dies idk what happens just that they both dead, he creates a world in his head where they are just friends and none of that happened, and their lives are just a story, when he regressed back in time his mind coped with the regression as him going into that novel as the mc. And since it’s not “real” he’s coping with the events really well so far so iheon doesn’t want him to regain his memory, once he accepts the split personality, he will then know that it wasn’t a novel, and all of the events he knows are about to happen already did happen already, he then might grow to hate iheon for all that’s happened, and iheon is afraid to loose him

As of right now what’s currently going on is, Huimin doesn’t realize it yet but the “book” he read and thinks he's gone into, is actually his real life, and he went back in time along with Iheon to get a “due over”, if you will.
Iheon has no clue Huimin knows the events that occurred due to his knowledge from “the book”.
Iheon, I believe, is unstable due to the fact that Huimins pheromones can't be fully developed unless he remembers that his “book life” is actually his real life however, Iheon doesn't really want Huimin to remember because he thinks Huimin will hate him and try to “off” himself if he realizes the pain past Iheon put him through.