Good point SEB. Improving without proper guidance would be very difficult. I think it's possible to get a better on the bag, but the subtleties of the science will be lost--and there's a wealth of knowledge to be learned. It's also literally impossible to learn defence without proper training. It's the kind of thing you need to learn through training and having someone throw punches at you, otherwise the basics will never fully sink in. For example, your jab hand will NEVER come back to where it should be until a trainer pops you in the kisser with his mit.

keep it up though. maybe you could open your own gym. If you build it, trainers will come.