Don’t worry. Most of us didn’t. Try not to understand the logic because it’ll just hurt your head.

I’m one of the few who saw that as in line with her character, rather than the writer creating tension just for the sake of it. From early on, when Se-hee spoke to Ji-ho’s mum at the wedding, he told her that Ji-ho is always clear about what she wants to do with her life, and she would never do something that makes her unhappy.

What happened with her and Se-hee was that he was very damaged by his past relationship, and couldn’t open his figurative Room No.9. So even though it was a tough decision to make, Ji-ho realised she needed to take a step back out of their cohabiting situation in order to find out whether she could commit to their relationship in its current form (with Se-hee not fully able to express his feelings for her). She wasn’t manipulating his feelings; she just wasn’t sure that she could see their relationship through, even though she likes him a lot.

I have explained it earlier on somewhere ondramabeans but don’t know where. To my own understanding, Ji ho broke up with Se hee intentionally so she could hurt him. Her character is like that, remember when she got so close to him because Se hee said ‘uri jib, our house’. She made breakfast for them always and even named the cat. That was when Se hee reminded her of their marriage contract and that she crossed a line. She became so petty the next day that she denied him breakfast the nest day. Puting this in context, explains the readon she left him more like a fit, for him to suffer and for her to see how e will cope without her.

Sorry for the long explaination but thats what I understood…

So, this is the sticky point of the show, where there isn’t much agreement. The show never really explained Ji-ho’s reasoning in a clear manner. There are beanies who think Ji-ho wanted to force Se-hee to evaluate his feelings and also force him to open up to her. Some beanies think Ji-ho initiated a break up just because… laughs?
But my personal understanding of her reasoning (and I am in the minority here, but I really felt like Ji-ho is similar to myself and her actions were nearly what I would have done) was that Ji-ho suddenly became scared of how their relationship had quickly progressed.
In her mind, her time living with Se-hee was a period where she was maturing both professionally and personally. Because she had done so with Se-hee by her side, he was like her support system, so she became worried that all she felt she had achieved wasn’t genuine. Ji-ho also still held some minor worries about her relationship with Se-hee, they were still working on how to effectively communicate with each other, because their personalities were different (this is in part why her decision to leave was not understood well by Se-hee, she didn’t explain in a way he could understand). So, Ji-ho thought that if she could spend some time alone, she would be able to figure out herself a bit, and return to the relationship feeling more secure that she was ready for the commitment.
Now, the phone call with her mother while she is “away” is what makes Ji-ho’s thoughts most clear, and also why she returned in the manner she did. Ji-ho tries to tell her mom why she left, but her mom tells her that the reasoning is BS, in a relationship, two people need to grow together and support and rely on the other, so Ji-ho’s fears of relying too much on Se-hee are unnecessary. Also, the solution to poor communication is to talk more, not to try and think it through separately. So Ji-ho returns to Se-hee immediately, because she hadn’t liked being away, and her mother’s words gave her the confidence she needed to put work towards the relationship.
I don’t know if I explained this very well, but I hope it makes some amount of sense? Then again, if I am the only beanie who felt they could relate and understand Ji-ho, maybe I am equally as difficult to understand! Also, feel free to disagree with my opinion, nearly everyone does!