I've been (hell, am) exactly where you are, and honestly, the answer is:
-Draw more lines. 'Clean' lineart is the culmination of a lot of really difficult skills. Contour drawing, line weight, perspective, gesture drawing, analyzing form... You just need to draw a hell of a lot more, and let yourself struggle with trying to get it right for a while. Find more artists whose lines you admire, and try to find out what makes them tick. It's not that they hold a pen steadier than you, it's that they make better decisions.
-You have a very 'messy', tactile style, where I assume you like to feel around a lot with lines as you draw. Don't fight that and try to make super clean pen lines -- embrace it. Even with that style, clean lineart is simply
a matter of skill
-Lastly, don't be afraid to cheat just a bit. Fake it until you make it is a great motto -- if you need to use the photoshop pen tool every once in a while to get some intricate, clean forms in perspective, do it, it's going to be more productive to put out a lot of good drawings than it is to agonize over one beizer curve.
ps: dont forget to make your canvas big enough to effectively support whatever brushes you're using. In photoshop some artists like to go by 2x monitor res, and I honestly like to go even bigger, because shit starts to feel weird and fiddly when a brush gets small.