I think I can at least understand Tiara's feelings.
She knows how everything was going to end end and is trying to change it, but was hit by the fact it could never work. Also she can't really walk to him and say shit like: "I'm sorry for making you do this but if you don't we can't be together." She doesn't want to lose Aqua, and to prevent the CG scene she realldy did ask him not to go to the sea, but the CG happened anyway. It's something she can't control. She can't talk to him about it because, no one is supposed to know "the ending", and also it would be telling him directly that she doubts his love, in which she doesn't. What went wrong was running away while the other girl was helping him - this was an extremely bad move.
Personally I don't think she's dumb or weak, but she isn't making the wisest decisions in newer chapters. It's because she's keeping her worry to herself, she needs someone to share this right now (instead of shouldering everything herself and give the Fairy King his chance), someone who is aware of the "ending" (yes I'm talking about Akari). And unless we're going down the Fairy King route, I want more interaction between her and Akari, since Akari apparently played through Aqua's route in the Sequel, and thus can at least share her knowledge on the route. (If we're going the Fairy King route, my bet is that it is the "hidden route" that Akari did not know about. Would hate it if it goes this way though, it's kinda frustrating).
Thanks to the translation team for the upload. This does have potential, I will be looking forward to the upcoming release.
Also that's one of the cringiest CG name I've ever heard.