A hypothesis: 

I think that if you didn't have a clear overarching key purpose to focus on singlemindedly at the moment, you'd do better work if (1) you picked one and made that the clear dominant focus of your work/creative life for the moment, (2) you made it finish in a rather short period of time (maybe even just 1-3 days), and (3) you set it in such a way that it was a process that was almost impossible to fail, so you'd have a relatively high assurance that you were going to succeed.

Not having a clear singleminded thing to focus on means needing to constantly evaluate priorities. The toll it takes, mentally, seems to be quite high. Sometimes it leads to people feeling overwhelmed and confused often.

A lot of this fades away once you get focused singlemindedly. If you're preparing for a huge competition as an athlete, the volume gets turned down on everything else. You might have other things you need to do, but you try to structure things to get them done as expediently as possible so as to be able to focus on your training.

I'm frequently asked for advice by people who are driven in a general sense, but don't know what they specifically should be doing at the moment. In the past, I gave advice that was not quite wrong, mentioning a few topics that would be worth spending time on.

But I'm starting to think it wasn't perfectly right either. Having a singleminded goal to work towards lets you bring all your attention and focus to the problem, and it cuts all the weighing and measuring and uncertainty. 

With that said, if you're picking something somewhat arbitrarily as your key focus, you'd damn well better make it a short-term goal. Think days, not weeks. 

How many times in the past have you started a new campaign or goal vigorously, and then had it fade away uncomplete?

This is okay, everyone does it to some extent or other, but it risks setting the bad habit of starting things without finishing them.

Actually, that's not quite accurate: starting things without finishing them isn't really a habit; it's the default state of affairs.

Rather, finishing things that you start is a fantastic habit that most people don't have. And if you set short-term goals with intensive focus on them, you can complete things, and thus be building this habit.

Which leads us to the last point -- if you're ambitious, you're probably setting goals that are results-oriented. Some amount of money, some achievement, etc.

There's a place for results-based goals, and if you had a perfectly philosophical disposition, then results-based goals might even be superior to process-based goals.

But, no one has a perfectly philosophical disposition.

Results-based goals can, and often do, fail. Before you're very-well entrenched in your domain of choice, you can't accurately predict the rates that people will buy or respond to what you're doing. The variance is too high, too much is out of your control, and you don't yet know enough mentally to be able to accurately benchmark the probabilities of various outcomes.

It's worth having some results-based goals, and the more philosophical you get, and the more mental strength you build up, the better they are. But in the beginning, start with most process-based goals. What process-based goals do is give you a relative assurance of success.

This means two things are happening: first, you're getting the habit of finishing what you start down. Second, you're getting all the mental and biochemical rewards that come with success -- which is also reinforcing the joy of finishing what you start, and making the habit stronger.

An intensely focused goal that takes only one day to complete, that you can devote your attention to singlemindedly on a single Saturday as if it's the entire meaning of your life... with a very high assurance that it will work out by you simply putting in the time and effort... can you see how this changes things?

You can only give all of your attention to one thing at a time. When you bring all of your character, skill, attention, and energy singlemindedly to a single task, even for just an afternoon, are you not working the very same habit and ability that lets you change the world?

Is there any real categorical difference between a single afternoon of single-mindedness, and stringing together a life of many days of this type of behavior to the end of changing the world?

Perhaps not, and it only takes an afternoon for you to test this for yourself. Do please come back and share with us your results.