Did I screw myself out of graduate school?
If you intend to pursue a professional career and not a career in research sector, I would say "apply with no hesitation". Do not let your doubts and cognitive biases prevent you from accomplishing all you deserve.

For someone in your age, grades do not matter anymore. Even for someone straight out of college, I would say that grades do not really matter. It's more about your experience, characteristics, and capabilities. Based on your post, I think you could easily prove those criteria to your future employers.

Regarding a career in research sector, grades might matter. Grades represents your academic discipline and how you methodologically attack problem statements. So, you need to do very well in the course of your graduate studies.