There are probably no simple solutions. But just in case, you havent already done this, just use a simple time logging tool and work in discrete sessions(20-40

There are probably no simple solutions. But just in case, you havent already done this, just use a simple time logging tool and work in discrete sessions(20-40 minutes). (Eternity time logger is one good app, there are others). Dont worry about being precise (in terms of minutes high or low, or exact kind of task, over analysis isnt good). Instead aim for consistent use. This allows you to work a certain number of rounds a day(instead of nebulous amount of productive time) and clarifies what is exactly happening instead on relying on internal psychological indicators of having done something or not. Also, specifically for the ADHD part, the problem with wavering attention is that when one comes back, 'loading' the context again takes a lot of work. So write a note at the end of each session on what's done or maintain a simple task text file(again, it is easy to over organize here, so keep things as simple as possible).