I was surrounded by clear ADHD cases growing up. They were more-or-less bouncing off the walls and constantly getting in trouble. I knew I wasn't like "those kids". But the inattentive types—those that can't focus or tend to hyper-focus on one thing to the detriment of others—fall through the cracks because they're not acting out. And, if they're otherwise high-performing, they'll be able to pull through their work with the occasional forgotten assignment or scheduling snafu. Teachers, bosses, even the affected themselves seem to think just need to concentrate harder and they'll do better next time.
But, telling someone with ADHD to concentrate harder is like telling someone nearsighted to squint if they can't see the chalkboard. And it's not a flaw in their character anymore than myopia is.
Anyway, I don't know your life or backstory, just that seems pretty familiar to me (tried pomodoro, thought of/started on a similar system before abandoning it in graveyard that is my ~/Projects folder). If any of this has struck a chord, I've found that simply changing my work circumstances have helped me immeasurably. Two years ago, I was working at home and left to figure out every choice and solve every problem in a pretty open-ended project. I'd spend hours reading papers, tutorials, and blogs with little to show. Now I'm in an office with an open floor plan and a team working toward a common goal with a time constraint. Medication would probably help me even more, but I exercise quite a bit and am wary of side-effects. Therapy would probably be even better, especially because, while my organization and execution at work has gotten better, my own tasks are about where they were before.
If you're interested, I found "Driven to Distraction" by Hallowell & Ratey to be a pretty good reference (although, if you get through the whole thing, you might not actually have ADHD).