I'm in my mid-40s and have severe ADHD and I've tried many many techniques and systems over the years. Over the last ~15 years I've come to evolve a set of systems that work for me.
I'm starting (in my "ample free time") to document them and in a series blog posts help people find systems that will work for them. My experience is that the best systems are the ones that have five characteristics:
1. They're simple
No complex patterns, no "we'll solve everything"
2. They require little or no task switching in the middle
This breaks my ADHD concentration.
3. They're forgiving if you fall off the wagon
You will always have bad days and need to restart. The system must make it easy.
4. The system must be very general, maybe even "too simple" but easy to customize.
There is a natural desire, especially in ADHD people, to over complicate, so the system must allow you to be as simple as possible, but then let you customize later.
5. They don't require any specialized tool (especially not an online tool). No system should be invariably tied to a specific piece of software or hardware. These may be excellent augmentations, but they should never be requirements.
Am I an "organized" person? No, but I'm far better organized than I was. Tasks rarely get missed now. I'm far more productive than I was (and I have stats to back up my assertion). I can almost always retrieve documents I need relatively quickly.
These systems won't change who you are, but they will assist you in being better at being who you are.