"Let's go jamming!" was a common phrase back in high school. We'd seen the experts do it, and they'd always seem to make magic each time they did. I was the best electric guitarist in school, and played in a band. So we decided that it was the right thing to do - you know, "practice".
Most of the time though, all we succeeded in doing was aimlessly dicking around for an hour or two. It was fun, though rarely productive.
We failed because of the failure to realise one thing:
(*) Improvisation is generative - a good jamming session is defined by the ideas that it generates, and doing that is HARD
You place a seed (which may be a beat, a chord progression, whatever), and then proceed in turn to iterate on different rules to create a new melody.
Achieving the required level of musical awareness takes years of practice. Even child prodigies don't get there until their in their late 20s, with the rest of us having to wait until our late 30s.
This isn't just true for musicians either; you see professors, designers, architects, etc only ever producing their best work in their 30s and beyond.
As a novice in a jamming session, you honestly don't know what you're going to play next. You probably know a couple of chord progressions and modes, but that C Mixolydian scale is getting awfully drone-y after 32 straight bars.
The expert honestly doesn't know what their going to play next either. But rooted in their subconscious are vast decision trees (with large branching factors, allowing amortisation of lookup essentially to O(1) =P) pointing them toward the most logical next choice.
Fortunately, you don't have to be an expert to experience and appreciate this fact. A simple heuristic: Whenever you're legitimately impressed with something you've "pulled out of the blue", it's time to stop and ask why. Celebrating some is great, but IME that is precisely the time whereby you're most likely able to consolidate your thoughts and grab a little bit of that subconscious
The thing is, that with enough practice, even a novice can make a song (just one) sound good. That's awesome! Mainly because it means that non-masters can still create value in this world.
Of course, that doesn't suffice for a musician. They not only must be able to perform their art in real-time, but they must do it again and again, day-in-day-out. Any failure on their part is directly experienced by the audience, who gives them instant feedback. This is usually subtle not reacting as positively as you'd like towards accentuations in your performance (not laughing at your jokes, if you'd like to think about it that way).
But most other fields aren't as unforgiving. All they demand is that you respect the craft, not necessarily master it. That means taking the time to worry about the kerning of all your titles, the formatting and choice of font for those finding screen-reading too taxing, etx. ie: Dot all the "i"s and make sure the present package is something worth consuming.
In a way, focus first on value, then on art.
I've been 4 years removed from high school, and while it's been 4 years of consistent practice, I'm obviously still no where close to being an expert. Fortunately, life is long (as long as you respect your body), and there is always going to be time to become a master. I just need to do my thing, and my subconscious will tell me when I'm there.