I can totally sympathise, I've lost that disc and slipped the one above it (I'm only 20! ) I don't train at the moment, I've just spent the last year in a wheelchair trying to get it sorted, but I too used to do TKD. Sit ups and stuff are great for getting a six pack, but you really want to train your core abdominal muscles, and surrounding skeletal muscles, because these are the things that hold your spine straight Sit ups mainly effect the upper abdominal muscles, rather than the deeper layer (which u want to work on) If you strengthen them, then you lessen the chances off irritating your condition because you've got yourself into a bad position whilst training, as they support your back. Learn how to use a gym ball (there's hundreds of excercises you can do), and try pilates, both of these are great for bad backs. Also because you're conditioning your skeletal muscles you should find that your TKD improves as your posture will get better, and your overall speed should improve. If you work out a certain movement is causing the problem, just don't do it. For me bending backwards is a big nono. Hope you get better soon