Road map for engineering degree later in life?

I am currently employed as head of engineering at a small manufacturing production shop. I have an impressive work experience and am insatiably curious, so I tend to learn things that interest me on my own, thus the ability to perform in supervisory roles in an engineering discipline.

I have reached a point where I have stagnated because I never got a degree. I would like to work for a large firm or production shop.

I have some concerns, namely that I don't have any university experience. My high school GPA is abysmal. Money is an issue, but more constraining is time. I have a full time job and spend hours a week volunteering - a priority in my life.

The biggest concern with school is the fact that I simply cannot motivate myself to do things for the sake of doing them. For example, I would fail a class on GD+T because I would simply not do the coursework as I already have a strong grasp on the subject. I would need an option that allows me to test out of classes for credit or take a different class to satisfy the degree. I get panic attacks in school, though this is manageable with enough medication.