I am looking for an Isekai manga; I might be confusing two stories together. We have our Heroine Protagonist who was killed in the modern world and Isekaied to a Regency world with Dukes and Kings. Since she thought she was the heroine and Isekaied, she thought she was going to be special, have magic, be the hero, etc. The first few years of her life, everyone thought she was a prodigy because she could read and write, but as she grew older and had no magic, she loses her favor and becomes depressed or something.
Now for the second part, it might be the same story or it might not. In this part there are two princes. The first prince is a military genius but his second brother sucks. The queen, who is not his mother, plays her political game and he is removed from the throne and his brother becomes king. He is sent to marry the daughter of an aristocrat that has a territory that is isolated from the capital. The daughter turns out to be Isekaied, so she has introduced a lot of technology to the territory. The province is very advanced compared to the rest of the world, an Atlantis sort so to speak. Now, the stupid brother King attacks another nation to prove his worth. So now the first prince and the Isekaied heroine are pulled into this war they don't want to be a part of. I think the soldiers that belonged to the first prince arrive at the castle at one point and the Isekai heroine almost kills them with her technology.