The Art of Problem Solving series of books is relatively affordable to an adult with disposable income and they all come with complete solution manuals: https:/

The Art of Problem Solving series of books is relatively affordable to an adult with disposable income and they all come with complete solution manuals: https://artofproblemsolving.com/store/list/aops-curriculum https://artofproblemsolving.com/store/list/aops-curriculum You could spend a year or two doing ~20 problems per day while scheduling review of definitions you've understood and problems you've solved with spaced-repetition software like Anki. Afterwards, you'd be prepared for any undergraduate mathematics curriculum in the world.