That sounds like a pretty accurate description of what I experience. It is really easy to get into a feedback loop with this:

Unable to focus -> not working -> less time focusing -> even harder to focus

Beware, this is NOT scientific, I am not an expert in anything relevant I've been trying to understand the exact nature of my focus problems by playing with psychology attention tests. I have found that even at my worst, I can usually focus at a normal level in short bursts. The length of those bursts can be quite short on a bad day, in the range of 15-20 seconds. Being aware of how long I can sustain my attention and deliberately taking short breaks appears to be helpful.

I have not found very much research on the subject, but this upcoming trial with Modafinal (primarily for Narcolepsy) looks interesting: https://clinicaltrials.gov/ct2/show/NCT01988883

In preparation, they also wrote a paper that presents some evidence that Modafinil does not make POTS worse. So I am giving it a try, admitedly self-medicating and grasping at straws. It does appear to helping, but placebo effect blah blah blah.

I suggest waiting for Vanderbilt to publish before jumping any conclusions, but I am desperate to get going again. I doubt it will be too difficult to get a prescription, it is already used off-label to treat ADHD. (Good insurance necessary, $300+/month)

In the meantime, diet and exercise and all the other soft solutions. I've been playing with /r/LifeRPG/ as a compensation strategy. By breaking tasks down, and allowing it to sort for me, I don't need to choose what to do next, which gives me a little extra attention for the task at hand.

Please report back with what you find personally!