Hi S.T.M,
I read your other thread "Why Can't I sing" and when you mention your favourite singers above I think one of your problems is you are trying to emulate them, and possibly singing songs from their repertoire, thus inviting a comparison and making you screw up. If you are interested in folk singing, there's plenty of songs available where people will enjoy the song without the need for a virtuoso performance. Real folk singing is more to do with the songs than the singer.