I think you should convey to your daughter that she can succeed in real life, at real things, at anything she enjoys and puts her mind to. She'd demonstrated she can do school work and class work successfully, and you've seen remarkable improvement and maturity in hoer study and time management skills. Good for her!
Your job as a parent is to help her navigate the more challenging parts of her life. And it's okay to have challenges--EVERYONE DOES at several points in their lives! So that's what you're here for now!
What you observe is that there is a discrepancy between what she's able to do in school when she focuses and puts her mind to it, and what the ACT scores demonstrate. What's up? Well, because you're a man of science and reason, you know it's not magic or voodoo.

Where there's a will, skill, and perseverance, there's a way.
So sit with her and evaluate the evidence. Is she applying herself to ACT prep? Where does she think the rough spots are--with the content or process knowledge? Or with test taking strategies? Or nerves? Something else? And then get on it.
I took a Kaplan GRE prep course and found it amazing, but that was 20 years ago, things change, and there's probably a lot of variability among instructors. Maybe a combo tutor (a good one, not just some shmo off the street) *and* test taking support would help?
Keep telling her, her job with your help is to do her best, identify road blocks, and find a way over or around them. You don't want the silly ACT to hold her back--you'll help her find a way to make her scores an asset, or to highlight for admissions committees all the other important and valid evidence of her competence and grit so she gets into a school she's excited about and can excel in.
PS: That school doesn't have to be an ivy. I went to an elite (top-20) kind of school; my sister went to a top tier liberal arts college, and my other sis went to a big state school. Guess what? We took different paths, but turned out remarkable similar! And the state school gal? Well, I'd say in many ways she outperformed her older sisters in the end. Though we're all happy, healthy, contributing citizens with families and pretty sweet careers.