No matter how much I practised this etude, i just can't play it in the right speed. I play the piano for 12 years approx. and it just didn't work for me with this etude. Does someone have an idea for me?
Frances Clark (a famous piano teacher) was asked a question about easy Chopin etudes. She replied (as documented in Questions and Answers) that his etudes only come in "hard, harder, and hardest". A graduate student in my piano program played a Chopin etude. He reported practicing 100 hours for a one minute performance. The Chopin etudes are highly difficult. The solution may be to: practice it for a few hundred hours in a disciplined manner (with metronome, slowly, high number of repetitions of short sections until perfect, etc.), or try it again in a few years once you are more skilled.
The first step is obviously to memorize the piece. Play it absurdly slow (semiquaver = 20; no, I'm not joking). Practice by writing down the piece, check where your memory is weak and train more those parts. When you are able to write it down completely, without looking at the score, you've done half the job.
Second step is being able to master all the technical difficulties in the piece (it's an etude, after all). What I mean is something like this: practice the first four semiquavers in the first bar with your right hand, at least 100 times. When you are able to play those four notes at right speed, then do the same with the other four, and so on. You may want to hold a book with your free hand and read it, like Liszt did, since this is the most time-consuming step. Like mjibson said, don't be scared if you have to put an absurd amount of time for a single bar. It's normal. The only thing you have to keep an eye on is your arm: if it starts getting tired, stop immediately: it means you're doing something wrong. Change position, try using less muscles to do the work.
Third step is putting all together.
The biggest challenge is the psychological one. Watching videos on youtube of 11 years old children playing this etude like a Bach's minuet may demoralize you. Seeing that after 10 hours of practice you only learnt that little bar may demoralize you. But the truth is that no one has learnt a Chopin's etude without putting in it days and herculean effort.