I mean id understand why she's scared imagine being engaged to a killer you never know when the killer might kill you. Also Arsene has lots of enemies so its normal to be scared since Arsene's enemies will likely target Regina because she is engaged to him.
Yall shes not dumb or whatever. She thinks that Diana is the only one that can lift prince's curse. The prince wont fall in love with Diana the story is abut Ancia and the prince not Diana seriously (ノ-_-)ノ
But im inlove with the roses like ITS SO BEAUTIFUL! (*´∇`*)