Not a software project, but I would try to learn mathematics properly and in depth. This isn't something I've had the time or opportunity to do.
I would get a stack of basic and undergrad textbooks, maybe a copy of Mathematica, maybe also a tutor, and just start at the beginning to see how far I could get.
I don't know where you found it but thank you sir! Very enlightening and helpful for those things you get rusty on. Calculus was 6 years ago now for me and just skimming over what is in this sheet helps a hell of a lot. Awesome!
A better understanding of math would directly impact your skill as a programmer. It might be worth it to float the idea with your job and see if they can help you with that. Then demand a raise because of your increased value.
I would get a stack of basic and undergrad textbooks, maybe a copy of Mathematica, maybe also a tutor, and just start at the beginning to see how far I could get.