I really admire your willingness to make sense out of her decision (I'm not being sarcastic).
When things started to go weird I made myself pay close attention to everything she said, because, like everyone else, I believed the writer knew what she (he?) was doing.
Maybe JiHo wants to be with him but really can't stand his parents, and since this is Korea, avoiding them is not an option. That's reasonable, I can relate to that. She should talk with Se Hee about this tho, not just run away and leave him in a puddle of depression.
Maybe she doesn't really know how she feels, since she knew he would propose but wasn't happy about it. That's reasonable, I can relate to that. She needs time for herself to think things through. But... she at least cares about him, right? So she really should explain what's going on on her head and NOT just run away and leave him in a puddle of depression.
MAYBE.... (I'm trying really hard here) since this is her first love and all, she got overwhelmed by everything that was going on and needs to back away for a minute. That's also reasonable, I can also relate to that. I could even buy that since she's inexperienced and all she just panicked and ran away on a whim. But... she didn't. She was thinking really hard and talked to a lot of people.
She wanted Se Hee to be crazy about her and show it. The moment he got mad she was happy. It was like everything she did was worth it because NOW she knows, without a doubt, that he loves her and he missed her. That's the only thing that makes sense. And I hate it. I also kind of hate myself for liking her pettiness on earlier episodes... It was like foreshadowing for this big mess.
I really wanted to love this drama... And that only makes it worse.