Don't just repeat the same things that didn't work for you in high school. If you had a hard time with textbooks and assigned problems in high school, you'll probably still find it frustrating today.

I'd suggest that you find yourself a project that you will be really interested in that requires math skills a bit beyond your abilities. Learn the skills that you need to accomplish this project, and then pick something else that will stretch you further. Use textbooks and online references to accomplish these tasks -- not as the tasks themselves.

If you're a programmer you probably won't have much difficulty coming up with projects to stretch your skills. Computer graphics, machine learning or science applications (among others) offer plenty of opportunities to use advanced mathematics.

If you can't come up with a project that you'd like to work on then you might ask yourself if you really need (or perhaps want?) to better your math skills.

Good luck!