Ah Jin's husband is clearly putting something in the tea that causes hallucinations and nightmares. To him, marriage is a game in which Ah Jin is a doll he plays house with. And behind the scenes, he wants to slowly break her mentally, just like he did to his ex-wife. He married Ah Jin not to protect her or help her succeed, but to destroy her. He didn't take pity on her past life struggles, but is exploiting those vulnerabilities to fulfil his sick fantasies. He knows she has no family, and he believes he got rid of her Jun Soo and Jae Oh, who were her only support system. By isolating her, he ensures that she will have no one to turn to, no one who will look for her, and no one who will care if anything happens to her by the time he is finished playing with her.
This might get a lot of heat, but I actually like Ah Jin as a character. Dramas typically portray the main characters as the "good guys" who are never in the wrong. When the protagonist is seeking revenge, their "perfect victim" image makes the viewer on their side unconditionally. This type of depiction is lazy and unrealistic because humans are not perfect. Humans are flawed, and no one is inherently or always a good person. Especially when seeking vengeance, morals will waver. That's why I find Ah Jin's complex character refreshing. In one episode, you may support her, while in the next, you might question your stance entirely.