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I think about this kind of thing a lot. Seems to me that everyone grapples with this at some level, though they may describe it differently. The word "should" is important. If you believe there are certain things that you "should" have done by a certain point in your life, or that you just "should" be doing this or that right now, then I'd predict some level of anxiety that might manifest as ADD symptoms or trouble focusing or just vague generalized frustration. Add that to a physically or chemically-caused difficulty in mental focus and I'd bet any of us would appear to be unfocused and anxious, maybe with a few panic attacks sprinkled on top.

So first, toss out the "shoulds." Like Yoda says, "Do, or do not, there is no try." And "try" is what we do about the things we feel we "should" be doing. Our parents and culture force-feed us our "shoulds" as soon as we can talk. Once we eliminate those, we also eliminate the stress-inducing tension caused by the opposing pulls of our genuine (and personal) needs and wants vs. a vague guilt-ridden sense of obligation toward some abstraction. Part of my own theory of focused, coherent thinking involves reducing stress, both internal and external.

Meditation tends to help because it reduces internal and external stress, and can even allow one to do the inner work needed to trim away those "shoulds." It can create new problems of it's own if seen as an end rather than a means, however. To just always clear one's mind, and then stay in that empty state for a long time (hours sometimes) is a very limited approach. Once the mind is clear and calm, then all kinds of things become possible- the empty mind is the starting point, not the goal. Ultimately a meditative mind state (characterized by Alpha or Theta brainwaves) can be achieved with one's eyes open, using internalized triggers built through repetitive practice. For example, if there is a specific sequence you always go through to meditate, then at the end of it you can program yourself a trigger that will nearly instantly shift your mind to that state if you use the trigger.

Specifically, an example would be a long relaxation sequence followed by one last, long, deep breath that you exhale completely and at the end of it your brain is in a meditative state. Do this often and every time, and you slowly build up a Pavlovian response to that breath. (You'd normally pay hundreds of dollars to hear this in a long-winded, flowery, and convoluted explanation on a comfy cushion during a weekend seminar in Malibu, so this is a hell of a bargain!) Now here comes the fun part- when you are going about your normal day, and need extra focus or calmness or higher performance you can repeat that long, deep breath and if sufficiently reinforced with practice it will trigger a meditative state in your mind. Seems suited to help with ADD if that breath can trigger a quiet, clear mind. Use that mind state to get your stuff done rather than just sitting there emitting greenhouse gasses with a serene look on yer face!

The point I'm after is that there are many interrelated variables that come together to create the kind of situation described in the original post, and no one thing is going to be the cause and no one thing is going to be the cure. Hell, maybe drugs are the easier answer. After all, Thomas Edison was an incredibly prolific and brilliant inventor (I'd argue he was really focused) and we all have his mind to thank for many modern conveniences, but his mind was amped to the gills on coke most of the time from his beloved Vin Mariani (not rails snorted off strippers, that was Ben Franklin). "Early to bed, early to wise..." my ass!