First of all, nice victim blaming you got going on there. Top shelf stuff.
Sunao has nothing (and i mean capital N nothing) to feel guilty about. What you are practicely saying to the guy who gets 3 timed by her childhood
BEST friend turned girlfriend to "get over it". Who are you to judge how someone allowed to process trauma?
You say you binged it but you apperently skipped most of the nuance during the process as well.
First of all, original title of the manga is "Good luck to you, bitch!" but EN release pussied out and made it PG13. This should give you the over all theme about how the story aims to portray Lemon from the get go. (Author is a woman btw, incase you want to go with "sexist man can't write woman" route). She is selfish and self-absorbed. At no point does she think about how Sunao feels or thinks. Can you imagine the mind set of a person who sees absolutely no problem confessing to their ex to get back together after cheating on them with 3 guys? Yes, she lied about it but Sunao doesn't know that and that lie is the very reason dude had absolutely no romance through out his teenage years. Oh yeah, he should have gotten over it as a 15 year old right? You are similar to Lemon in that sense. What a joke.
She showed no intention to clear the air so far as well. She didn't even remember how or why she lied during their break up, up until Sunao reminded her when they met again. She apperantly completely moved on with her life and if they haven't met by pure coincidence, she would have never contacted him.
Again, story makes it a point that Lemon is not right in the head, emontionally. Even the middleschool flashback chapter spesificly shows that she is still a massive pushover and didn't really change at all though she tries to convince herself otherwise while contradicting herself like 10 minutes later. And the imagery showed during the flashbacks implies that some girl in the class was also interested in Sunao and that's why she break up with him. Yeah, really.
Shitting on Lemon is not a "misogynistic" thing. She is a highly flawed individual and selfishness is one of the most hated character traits in stories. So readers tend to lesh out to those characters. I know you like your buzzwords but it doesn't really apply here. Not liking a female character doesn't make anyone sexist so get over yourself first.