disclaimer: self-promotion ahead, highly relevant but still...
> RE: resources for filling in the gaps
I recently published a book titled No Bullshit Guide to Mathematics that has precisely the goal of reviewing concepts form high school math for adults. Context: I was a private tutor at university for many years, so I know how common it is for university students not to remember anything from high school and struggle a lot, even though a few weeks of review would bring them back up to speed.
Once you have the high school math review done and solved some exercises and problems, you'll be in good shape for the other two books in the series No Bullshit Guide to Math & Physics, which covers mechanics and calculus, and the No Bullshit Guide to Linear Algebra. You can easily find links if you search for them and see reviews on the amazons.
> RE: resources for filling in the gaps
I recently published a book titled No Bullshit Guide to Mathematics that has precisely the goal of reviewing concepts form high school math for adults. Context: I was a private tutor at university for many years, so I know how common it is for university students not to remember anything from high school and struggle a lot, even though a few weeks of review would bring them back up to speed.
The book's websie is here https://nobsmath.com/ and you can see an extended PDF preview of it here https://minireference.com/static/excerpts/noBSmath_v5_previe... (see my other comment for links to the concept maps).
Once you have the high school math review done and solved some exercises and problems, you'll be in good shape for the other two books in the series No Bullshit Guide to Math & Physics, which covers mechanics and calculus, and the No Bullshit Guide to Linear Algebra. You can easily find links if you search for them and see reviews on the amazons.