Sounds as if you need both self-prep for content and a few meetings with a professional ACT tutor for test taking techniques.
Buy a copy of The Official Guide to the ACT. A previous year will be cheaper, and it has mostly the same tests. Then you do a section or two every day, do the corrections, and self-teach if you get something wrong. When my kid was self-prepping, they would do a hard one in the AM, and an easy one in the PM. For my kid, the two English were easy, the math was the hardest, and they were doing the science just to familiarize themselves with the manner in which the data was presented. My kid also got the two book series The Best ACT Math Books Ever (they’re available used for cheap) and used that to self-teach the math they hadn’t mastered or had forgotten.
After you’ve gone through all the tests in the book (I think there are 6, plus there is also at least one free one on the ACT website), there are retired ACT tests that are legally in the public domain that you can find online and download.