Read through, and do all the exercises in, one textbook each for:
1. Calculus
2. Linear Algebra
3. Abstract Algebra
4. Analysis
5. Topology
6. Probability Theory
7. Number Theory
...more or less in that order. Make sure your calculus book covers single variable and multivariable calculus. Supplement with applied mathematical statistics. Do that, and you have the equivalent of a mathematics undergrad (as far as relevant courses are concerned).
You could even do this with something like UIllinois’ NetMath program, or some courses on Coursera. You can swap out Number Theory for Complex Analysis or deeper Probability Theory and it’d be more relevant.