University of London does post grad maths diplomas . They are solid and somewhat structured for self study. They are also cheap .
Aside from that find yourself a tutor over Skype and crack on.
Just be sure you know what you’re getting yourself into. Unless you are one of the tiny number of people that have a genuine interest in maths, be prepared to slog through things you find senseless or boring in order to get to the prize. Further, and I think this is very true: there are no shortcuts. If you can implement something like mastery learning you will be all the better for it and if you don’t you may just regret it.
Finally, think really carefully about how you’re going to retain the maths you learn because if you don’t you can be sure that a month after your exam you won’t remember a scooby.