@fallenblue I remember it being stated that this world is a Mary Sue-esque one, and as a result, a ton of things are really weird (like the sister of ML's weird situation, how the hell the FL had a cheque just on her, etc). It's also been mentioned in comments that there's actually mention of the writers in the epilougue and why the characters are so strange apparently?
But basically, she fit the bill for what he likes, powerful and strong women, and since this is a Mary Sue world he fell in love comically fast.
FL probably wasn't in the best state of mind. Somehow she ended up basically under someone she heavily bullied. lost her dignity a ton of times, got abandoned by ML, and then learned her real parents are poor, which rips away the basis for a ton of her pride. Considering this is, again, a world where not much makes sense, she might've just lost it.