I see many people bashing the FL ... I think it's understandable because the main conflict of the story is the FL's difficult personality. She has anxiety and self-esteem issues due to her family relationship with the grandfather/aunt. You can see clearly even at a young age, she couldn't explain well or even given the chance to do so (ie gift to her grandfather). Her personality was partially shaped by her family experience.
She grows as the story develops. I think after 19, the story becomes more interesting as she started to change based on her experiences. The ML didn't give up on her because essentially, she's a good person ( although not an ideal GF at first): she doesn't do illegal things, she makes sacrifices for people, and you know you can count on her help.
In real life, I have seen women like this FL ... very demanding and difficult but the partners still put up with them because they see qualities that I obviously missed.
But she does have issues with treating people with empathy. Again she will grow ... if she's perfect, we will have nothing to talk about.
For me, this story is watchable (although with some plots and character development issues). It is watchable because the ML CXX is a good actor. I remember that I thought in one really cringy scene that wow, if the acting isn't natural and good, it would have been very difficult to continue. He delivers the scenes charmingly but very watchable.
The weakness of this story is that the writers make some unconvincing storyline but it's saved by good acting. So I would say, it's a watchable drama. However, if you cannot stand unreasonable, unempathetic female leads, then I would say drop the story.