You are discouraged by other people being better than you?
Enjoy being discouraged in EVERY endeavour you make.
If watching other people stops you from working, don't watch other people, because there are medical conditions which enable people to replicate things to a degree of accuracy, you would be hard pressed to match, as demonstrated here:
http://www.youtube.com/watch?v=cU9Cp5tIN2A&NR=1
The video you linked to and other videos like it don't teach you how to paint or give you a scope of how long it took to make, it is 8 minutes in length, slow the video down to the 1 hour and 20 minutes then we will talk about what is possible and how good it is, because the speed at which you see it come to life isn't entirely representative of the reality even if it is to a different scale.
The person in the video you linked is evidently working to a vague reference, he or she knows what the outcome is going to be roughly in both scope, angle, layout and size, but might not entirely in great detail. That is why they are able to lay down the basic foundations and then determine what they want to be wherever it will be. Much like Bob Ross used to say, "I'm just going to paint a little tree" it doesn't have to be anything special because a lot of lesser details make one big picture.
If you want to make a detailed picture of something, you are going to have to be patient, working your way across the image until it is complete.
In summary.
In order to solve your problem.
"Everytime I try to draw something, I go on Youtube watching Speedpaint or Speedart-movies"
You might want to swap the speed painting videos for, perhaps a tutorial video?
Also don't watch others whilst you work.
