Ah, well I'm often enchanted with many enticing stories on how well mangaka carry on in their careers, and one of the many stories involved tells how little work mangaka often have to do. In otherwords--while they may still work very hard, mangaka probably don't work as hard as a lot of people think. They've worked tremendously hard to get to where they are now, and have often had to do a lot of other things before actually becoming a mangaka, but the actual mangaka rarely make things as detailed and perfect as the finished product.
The final product of a manga is a team effort. The mangaka draws--often sloppily--the outline of the entire manga, and other future mangakas working their way up, take the role from there to edit and design certain things. While some mangaka do all their work on their own, not a lot of them do, so don't put yourself down if you're unable to draw as perfectly as they do... because the final manga is the collective efforts of many great drawers, not just one.
If you'd also like to draw that perfectly on your own, my suggestion would be to edit, practice, and draw as much as possible until you think you have what you need. It wouldn't be a bad idea to get friends involved in helping as well, or even go to school for the art. Whichever route you take, remember it doesn't have to be as hard as it looks~