The FL was an orphan, had to struggle and work extra hard to overcome her disadvantaged start in life. Due to her courage, resolve, and overall strength of character, Seohyun manages to survive for over ten years in a special forces group made of normal people, in an esper/guide world with supernatural monsters. At an advanced age, Seohyun presents as a guide. An average guide. The government agency she works for tests her guide rating and also discovers that she is a 99% match for a S-rank esper (the ML, Ciel) The two are quickly paired up by the government as a work pair to fight monsters. Since Seohyun is only a B-rank guide, the government's decision pisses off a lot of high ranked guides who would have loved to work with an S-rank esper like the ML. The most pissed off however is the original FL, Seo-yoon, who starts trying to break these two apart.

After working together for a while, the ML proposes to the FL. She, who admires his looks, personal ability, and demeanor, and yearns for a family since she's never had one (orphan) is flattered that her handsome, powerful coworker wants to mary her so she accepts and becomes his wife. Slowly, her initial infatuation grows into love, but it is an unhealthy love because her and the ML's behavior & expectations don't match up. On the surface, the burden of feelings is placed only on Seohyun. The ML, who is also in love with Seohyun, doesn't want to admit his feelings because he is already feeling trapped by his circumstances in this new world (being used as a weapon by politicians and rich people, having almost been subjected to a forced brainwashing/imprinting by the Korean government that Seo-yoon and Seohyun are working for)

Meanwhile, the original FL, Seo-yoon, doesn't stop trying to drive a wedge between the two, maliciously working to lower Seohyun's self-esteem. At a low point in her life, when Ciel loses control of his esper powers, Seohyun decides to do her final duty as a guide and calm him down, even knowing her survival is unlikely.

When the ML wakes up from his rampage, he realizes his wife sacrificed herself for him, is unable to accept this, and the subsequent power surge rewinds time, only this time, Seohyun is in the body of one of the natives of his original world. This is where the story starts.

She has an orphan background but is a former military special squad/elite member

From the limited backstory that we have, he is, at most, harsh with his words. He never cheated or abused her-they just lacked communication

I understand that there was another guide trying to get between the two of them but nothing has indicated he went with her. Yet this moronic female lead, accepts a book from this rival, who she should have at least disliked

is an original occupant of the empire that she transmigrated to. He has also regressed back to when the saint first appeared. He was also tricked into going to the FL’s original world (by the saint who originally came from there) and is desperately trying to get back. He loses control of his mind and abilities because the saint said oopsies, I cant send you back.

his perspective of his wife doesn’t make sense. It’s too positive

He says that she will ignore those she hates and will give everything to those she cares about. But like previously mentioned, she had no immunity against her love rival, the saint, for her husband and takes the saints actions over her husband’s.

Early on when she and her family go to the palace to celebrate her debut, she repeatedly states that they are staying at a run down hotel for commoners. I’m fine with this level of disregard except she talks about how close her family is with their people and how much her family cares for them. But they stayed in a “run down inn for commoners” which pissed me off to no end. F*ck you and your frugal nature if you somehow now think of yourself as better than everyone but still want to be considered a saint