Some where...some time...I overheard a mathematics teacher (or two+) say they wished they could retake their college mathematics courses...as now they might understand them and even learn something of value from them.
M.I.T. to the rescue, in most cases. That is, MIT Open Courseware
to the rescue. Each course offers a free syllabus, suggested readings (and links), free lecture notes, free exams, and free videos to support a myriad (i.e. 2000+) of courses offered by professors at MIT. Plus, no registration required..and no cost (other than your time and energy).
What a deal....now, you can revisit the courses you would like to retake...or even take for the first time. Specific to background for secondary mathematics, some options to consider are:
Many of these courses are taught by "biggies" in the field...for example, they might have written the text you used (and perhaps now is gathering dust on your bookshelf).
Be forewarned...these courses are on-line versions of past MIT courses...and remember the nature of students at MIT. That is, the courses may be difficult...they may be abstract...they may be applied...but they might also be interesting!
And, if not a mathematics course, their catalog offers a wealth of other courses as well...such as Computer Games and Simulations for Investigation and Learning or Paradox and Infinity or even American Soap Operas.