I know what you mean Barry - how many times have I deluded myself into believing I understand the theory behind something, only to be proven wrong by trying to apply it?!
As the saying goes: "In theory there is no difference between theory and practice, but in practice there is"
sarjumaharaj, I agree entirely with my earlier co-conspirators' remarks... MrFlibbles comments about mucking about and brushing up on the theory as needs be is the formula that works for me. I have boxes of broken projects/half baked ideas/experimental pursuits that failed in their intended purpose, but taught me heaps nonetheless. The theory is *so* much easier to learn (and truly understand) when you believe it has a purpose and you WANT to understand it because it'll let you do something you otherwise can't.
The trick (that most of my students report works for them too) is to find something that motivates *you* to tinker and give you some practical reason/conceptual framework to attach the theory on to. Interestingly, practical skills & theory gained in seemingly disparate areas often aids understanding in other areas, so don't feel you are ever wasting your time (unless you blow off studying for your final maths exam to finish your guitar amplifier like a friend of mine did
If you're interested in cars, go and try to build an ignition system controller (and learn what you need as you go). Audio? Effects pedals/amplifiers. Astronomy? A radio telescope! Anything will a) motivate you, b) provide intuition/insight into the theory, and [importantly] c) give you a substantial advantage over your peers in the eyes of a potential employer. A person who tinkers in their own time demonstrates independent learning and a commitment to a task that extends beyond the close of business.
So go pursue/discover your passion and study/life becomes more fun!