remige. i actually have that exact problem. it's either i spend hours on something, and it looks like a masterpiece... or i can't do anything at all. the perfectionism is more of a burden than people think. sometimes it stops you from acting at all.
That problem is the only reason I ended up with my skills. I kept going until I got it right. It took me years and years, and thousands of sketches and drawings. I have many peers in the illustration field, and all of them have the same biography. Keep going until you master it, and be ready to work for years until you do.
Talent is simply that part of yourself that says, "No, this isn't right yet."
Skill is the part of yourself that says, "This is how to do it."
Mastery is the part of yourself that says, "My skill will not rest until my talent says it can."
In all honesty, talent isn't an innate ability to perform a skill. It's just the ability to see a need for improvement where others may not. This is the case with any skill that people are considered to have a talent for.
While most people can draw a picture and then realize that something is wrong with it, few people can 'see' what it needs to be right. As NFJ's, we are uniquely equipped with an advantage in this area of cognition for artistic endeavors. Fe gives us the drive to express ourselves outwardly on an artistic level. Se sees the problem. Ni leads us to the solution. Ti helps us reason how to solve the problem on an abstract level.
To any of you who feel that your work is 'not good enough', I say this. Keep at it, and know that you have the capacity to be among the greatest artists in your fields. The only thing it will take for you to improve is dedication to the art. You have all the other talents you need. You can be truly great if you will do nothing more than devote the time and effort to do so because your cognitive abilities grant you all the 'talent' you need.
And finally, as NFJs or any other MBTI type, this advice is just as valid. Developing artistic skill is a great way to improve your Ni, Se, Fe, and Ti. While NFJs have natural advantages with these functions, we by no means have the market cornered. My father is an ISTP (also Ti, Se, Ni, Fe but with Ti and Se dominant) and developed a profound amount of artistic ability in illustration very early in his life before becoming a martial arts instructor (the Ni/Se combo I've mentioned in other threads) and eventually an R&D engineer for the government. His work is very precise and technical, but still very artistic and of very high quality. My mother is an ESFJ (Fe and Ti, but Si and Ne) she is also an accomplished watercolorist, who has had showings in galleries around the world. However, my father and I inspired her to develop her artistic skills, and I was able to watch her develop her Ni and Se as I was growing up. While it was more frustrating for her than it was for me, she kept at it and developed a great degree of skill. But, at the same time, her personality began to change into that of a more balanced and healthy individual as she enhanced her Ni and Se.
On a related note: I have no musical abilities whatsoever. I gave up before I ever developed them.