May I ask what do you mean that he never cheated? Sorry I didn't read but I'm really curious about his relationship to that red hair girl.
Oh, I can't explain it properly since it has been some weeks since I read this, but basically that red haired girl lived in the countryside when a new stablekeeper was hired (who was actually the FL brother, Glock, who was hiding from ML) and she got pregnant from him, but lost the baby from shock because of her father's suicide, Glock abandoned her and went into hiding again, and when she was about to be sold off because of her father's debt the ML saved her, which btw he was actually there looking for FL's brother to kill him, but since he discovered that this girl had a relationship with Glock, he thought that he could get him by having her at his side, she is just a bait, BUUUUT seeing how the duke's (ML) wife is a timid person and their relationship started with a contract she thought she could replace FL and become a duchess, not to mention she thought that he was looking for Glock to kill him because of HER (narcissist much? Lol)
Unlike some in the comments, I actually like this manhwa, the FL isn't a "playvictim" she IS a victim, she was kidnapped, abused and almost raped by her "brother" so it's totally understandable why she don't trust that the ML loves her. These reincarnation stories where they all change the plot is mostly fantasy, not realistic, if someone is abused and didn't have the right to education most wouldn't be capable of escaping their abusers even if they had their past memories.
And I also like this manhwa because it got a good pacing and clearly shows the ML side. In the past life he loved her before she proposed to him but because she threatened him without explaining her situation he felt incredibly betrayed (We both know nobody would be happy to be threatened with the fact your mother went crazy and killed your abusive father, even if you liked the person who threatened you) so I don't really blame him that much, especially that he never cheated on her too.