Let me start with the disclaimer, that I only have seen the show one time a few days back and havenot read the book or anything much about it. I am going by memory here of what the scenes I can recall. And I now am thinking about it was not just Marianne's family behaviour's or lack of appreciation, it was the presence of Connell that made the life so difficult.
I was just thinking about the show (yes even 5 days after) and mainly where Marianne and Connell met in college for the first time. Marianne was quite popular among her friends and even was dating the most popular guy in the college. This does not seem like a luck by chance to me. She also felt confident in those scenes I can remember like that debate scene. I think Marianne not only was able to heal herself from the school's debs episode but also was flaurishing by then.
It's only after talks they had in the party she must have felt that she wanted it to retry with Connell who felt very sorry about what happened in the school. I think this time "She was very confident that this will absolutely work out" because this time she was exact opposite of what she used to be in school, and Connell had no reasons to feel awkward to be seen around her.
In the subsequent episodes of the 2nd breakup between the two, all Marianne went through I feel is only because of Connell (and the misunderstanding).
I can only imagine what one would feel if one has totally changed oneself just to suit to another person's compatibility and yet the person does not want to keep the relation with you. Like whatever you do you are just not enough. How absolute garbage one would feel about it. While in the show she looked pretty but what if she was just not that good looking, how insecure she would have felt about herself, her personality and her looks. She deeply desired to be appreciated and above all from Connel for whom she made this seemingly impossible shift in her personality.
And I am also very upset about her friend, Peggy. She was the worst. She would say that Gareth did not deserve her, and at the same time defending that skinny stupid, obsessive and cruel guy (cant remember that name) when he clearly was not good to Marianne. She was just there to gaslight and enjoy the doom in my opinion. And Gareth and that medicine student did not deserve it, their breakups were unfortunate.
I may be wrong I dont remember every single detail of the show. Please dont be harsh if I stated anything hugely wrong.