I've got reader's block

I started to write a book about a messenger who gets sick of being run over by idiot drivers so goes on a violence spree, which leads to him getting sacked, and begins to ruin his only potentially successful relationship with a girl, etc. but my writing skills are poo. I even got a 'novel-writing tips' book out of the library! I still have all my notes and all that but I struggle to make a story believable.

I don't believe it.