I, too, will be writing a GPA addendum. Generally the most important points will be:
1) Yes, I did have a legitimate reason for doing what I did.
2) (If #1 is actually false): Yes, I did learn something constructive from that stuff I did.
3) I am a better and smarter and more diligent person now, I have evidence to back that statement up, and no, I will not screw off in law school.
Losing your way, so to speak, is not uncommon and not necessarily a bad thing. If you've, er, "found" yourself as a result it probably means you won't be inclined to engage in any funny stuff that might make School X wish they'd never accepted you. But the most effective way to demonstrate that is by having a killer PS, having LORs from God, and having indisputable proof in the form of improved grades.
From what you've said, I would ask: why didn't you like school? What makes you think you'll like law school? Why did you finally come back and finish the degree? These would be reasonable questions to attempt answering in the addendum.
I'm willing to look at your draft, if you like. You can PM it any time.