Advice from someone who was in the same position: Take a class. Multiple classes. Go sign up for a Mathematics AS at your nearest community college right now. You will never know enough of what you don't know to learn this stuff on your own. A lot of it is just doing the painful repetition work of practicing problems over and over again, which is hard to force yourself into without a "coach" pushing you. Having a cohort of students to work through problems with is also priceless. And the drive of having accountable grading will keep you at it regularly.
It can be a bit awkward at first feeling stupid not knowing what a logarithm is in a room full of 18 year olds. But it's the only way to really get there. I went from high school dropout who didn't know how to add fractions to passing calculus in 18 months.