After dating for a while and having a great time together, the MC eventually proposes to the FMC, but rather than giving him a clear denial and tell him how she feels, she just leaves and then basically starts ghosting him.
Later on, when they run into each other at some point in the future, she finally explains that she thinks the protagonist is in the prime of his life and that she would be holding him back by marrying him. At no point does she actually ask him about his opinion on the matter or try to give the relationship the benefit of the doubt to see where it'll lead them; she just decides that this is the way it has to be and then leaves him.

Two years later, they meet up again.
During their time apart, the protagonist's life has more or less crumbled entirely. He got married, but his wife had a miscarriage followed by a mental breakdown that turned her into an empty husk. His father-in-law died, his own father is in poor health, his brother's marriage is falling apart, and there are also some issues related to money that we aren't given any specific details about. In any case, you get what kind of situation the MC finds himself in. Rather than enjoying "the prime of his life", like the FMC talked about, everything turned to sh*t for him.
Meanwhile, she has been doing great during their time apart and even got married to another person.
Let that sink in.
She was really adamant about not getting married to the protagonist, to the point where she didn't even care about what he wanted out of their relationship and his life, and then she not only hooks up with another dude, but also marries him during this short time span.
The only thing the protagonist gets out of this encounter is the FMC telling him that he's a good person, and then she goes back to her seemingly perfect life while he has to return to his life of misery.
The End.

And not everything ends well"

but unfortunately he fails in his understanding of situation

had had so much hardships in her life that had to take medicine, relieve stress in Gym and immersed into work

First of all, did Manager Ahn even once said that she loved Son? No. Why is that? Because for her he wasn't a lover. He was a beam of light in her mundane hard single-mother life. Like oasis. A man with the characteristics opposite to her husband, with no link to her family or relatives, her past. That is why she was shocked when he proposed. She wanted him to stay the way he is. But he tried to become someone she already had. The notion she didn't desire.

She said it herself during their stay in the hotel bar. That "everything happening now is illusion, another universe". This was the only place where (in her world-view) they could be this close.

Manager Son healed her with his presence. She stopped going to gym, take medicine and even left the job. All the things that were connected with her stress. She saw in him something that makes her life happy. As long as he stays "good guy" - she will be happy, like a talisman. In the last chapter she almost broke when she saw Son's expression (when she said about new surname). But the moment he smiled she felt relief. So for her he is like a depiction of happiness. She says it herself. And she didn't see him happy with her. Her egoistic conclusion.

This is the main problem - egoistic thoughts of WMC. She thought that he wouldn't be happy with her. But at the same time she didn't think WHAT would make him happy.

All the biggest character changes that happened in the story were those of Ahn. And the main character turned out to be a support character for her happy life. And more so: she left her whole mundane life to him (even writing that she liked to do). He became miserable guy who continued to leave her life, workaholic with problems in personal life. She just became his doppelganger. That took his normal life and left him with nothing but hardships. At the same time she really thought (i hope) that it was for the best.

My point is. That if you look at this manhwa like at any other romance comics - certain decisions of characters and the ending itself are bad. But if you try to understand what happened and are ready to "see some unpleasant turns"... It's not bad, just complicated

bittersweet. Also the sex scene was the first one to make me sad.

If someone could please explain to me noona's reasons for not dating the mc I would appreciate it. It felt super vague (maybe I'm just malding) and I don't get it, other than something about being a burden because she's older and has a child (which I think is dumb). ????