you are saying this because you dont know why she is actually doing this. I've read the novel and i must say she was not doing because she wants stay true to the game plot. But because she wants to save Cecil and everyone's life.
(spoiler ahead)
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In the game if the heroine went any other route than the Prince, he becomes more emotionless and later becomes a tyrant where he goes to war with other countries and kills thousands of people.
There truly is a thing called the 'Fated Maiden'. But it is only for him that's why he didnt find anything in his research.
In the game only the 'Fated Maiden' can heal his heart. They form a soulmate type bond as Cecil felt love only for her which prevents his heart to be consumed by the darkness.
She has not made it abundantly clear at all. She just spouts that nonsense all the time without any explanation which makes her look and sound very childish. She always days villainess this and that and takes no one's opinion into account and tries to force her narrative of what is going to make them happy . I have not read the novel if it exists but in the manga she never fully explained what's at stake. If she is going this for the good of everyone then she should explain everything. She gives snippets of the story which makes everyone frustrated and annoyed, proven by the prince's reaction in the latest chapter.
Yes, not everyone is going to buy the "I l how all this shit because I'm reincarnated and this is an otome game" but she could've found a better way to deliver the message. I've read hundreds of mangas in this isekai/otome game/transmigration genre and I think she mogj5 be one of the most annoying MCs ever. I don't know if she'll redeem herself in the future but for now, she is unbearable to watch.
because she really is a hyperactive dumbass with a good heart. Also, she only has a few vague memories of her past but that doesn't mean she is her past self. She is herself with a memory of the past self. So she's really just being a child.
You can't always assume everyone will be a Mary Sue and always think of some WOW plots to end world calamities. The prince is the one filling that role.
Wouldn't it be boring if all of the characters are all fantastic genius people? Where will be the diversity? And there's already a lot of manga/novel that has done that.
Her character is built to be a fresh take on the whole villainess reincarnation to otome games. She wholeheartedly takes the role of the villain in the dumb otome game cuz she wants her family and friends to be saved.
I guess, if you're a child with sudden memories of a game where your mother died, you too will be put into a frenzy. I'm just amazed that she ain't mentally disturbed yet from the bad memories. She's dealing with it in her own way.
No one is perfect even in novels.
I sympathise with your frustration but there’s a thing you need to keep in mind. Most of the manga so far was told from the prince’s POV. So if Bertha refused to let him know anymore than she had done, he has no way of knowing what exactly to happen in the game’s storyline and that conveys to us reader. I agree that she’s not the smartest character ever, but her airhead-ness is supposed to be her appeal and according to the spoiler above, what saves the prince from turning evil so it’s what works for this story.

Sometimes (most of the time) she pisses me off a lot. I forgot how old she was before she was reincarnated or if it was never mentioned but unless she was 3 or 4, she is a complete dumbass. I've read a lot of stories where the FL is portrayed as "adorably unaware of others' emotions" and sometimes, it's cute but this ain't it chief ! She just gave up without living her life. And it isn't like those stories where the FL is like "fuck it, I'll accept this and work towards my ruin and then live life freely", she is totally saddened by it and odesn't have another plan so basically she is driving herself towards this "martyr end" and it's very stupid. At least have a plan B or something. This isn't cute at all, it is stupid, so stupid that unless she is mentally challenged, none of the shit she has done so far can qualify as human-like behaviour