there are fundamental math that appear in all other math. i think calculus is one such math, and perhaps probability (even more true given you’re pursuing machine learning). for both subjects i almost self-teach using calculus made easy (https://www.calculusmadeeasy.org/) and probability for the enthusiastic beginner (https://scholar.harvard.edu/david-morin/probability). both are fairly self-contained although the sometimes take shortcuts that can be dizzying at times. but since you’ll proceed at your own pace, you’re about to make a diversion to investigate, further, why they were able to take those shortcuts. check them out and see if they help. otherwise i’d like to know what worked for you :)