They skipped over the explanation in the anime, but there are special skills that are ultra rare (the king is king because of one of those) and she kept her Grimoire skill hidden. I think the explanation was that as the future queen consort she should not overshadow her fiancé, which she automatically would if anyone knew about her unique skill. As far as anyone outside her inner circle knows, she has just inherited her family’s ice magic.

As to why she lets everyone walk over her: she was raised to be that way. The question should be rather why does she switch so fast to the revenge side. The light novel makes that transition more gradual during her time in the cell and having her inner monologue helps that transition as well. The anime makes it look like all it took was that little speech fro her maid and she flipped a switch. But I’m okay with this to get the setup out of the way as fast as possible. In the end it doesn’t really matter why she is on the war path, the interesting part is what she does on it.