I couldn't stand a regular 9-5 job. Almost 4 years ago now, I was fired from the last I worked at and haven't looked back. In the last 30 days I've worked on about 16 different projects for various clients. That's typical for the last three years. Many of those projects are long term, multi-year, projects but none of them occupy all my working time. I work on them, take a couple days off for the client to review and give me feedback, and then I repeat the process. It's very rare now that a single project takes up all my time for more than a week.
You just need to embrace your ADD and find diverse work to do. Then you can distract yourself with productive things to do.
The other thing to do is start getting paid hourly. If you're not being productive you can just clock out and come back when your brain is ready to cooperate. Being paid for 8 hours whether you do nothing or something is probably not helping.
The other ADD friendly thing for me is having a backlog of tickets. If I have one thing to do, my brain tends to shut down because it's bored by the prospect of doing one thing. I need to be close to overwhelmed with tickets in order to maximize productivity for extended hours.