I can relate to your post... I'll find a track with a fantastic vocalist and think, "fu**, I'm not gonna be as good as this or as creative and my lyrics suck and my range is too limited..." and so on and so forth. Then, I listen again without the intention to add (because i suck, remember?) and start thinking, "wow, what a beautiful creation... cool lyrics too... wikiloops is so cool!" and at that point, I'm ready to jump in, knowing that sharing (not comparing) is caring...
This process isn't instant but the sooner i can turn my fear-thinking into positive thinking, the sooner I can sing again! It may be a few minutes or it may be a few months... We're all unique (musically and otherwise) and have our individual gifts to offer so the whole comparing thing is obsolete... This is something I struggle with in most areas of my life so thank you for being courageous enough to open this conversation.