Trying to help but a question such as:
Is there an on-line, human based math course I can take to get Julia to stick?
Is not specific enough as it depends on what you are trying to achieve. What branch of mathematics? Primarily, you seem to asking for a math course, not a course on Julia. Perhaps try a math forum and if you find a problem you want to cast into Julia, I am sure someone could help.
My fav book on Julia has no math whatsoever, but it teaches you how to write good Julia code and how to take advantages of the Julia design ethos (e.g. traits) . : Design Patterns and Best Practices with Julia
I also think, if you are a seasoned programmer, the Julia Documentation is amazing.