It's really hard to stop the jabbering in your head I know, Trish - you have to get to a point where you don't even let that start...and that takes a while to master - or it did for me.
Till then NO is yr friend LOL.
I think a sense of purpose and some self esteem go a long way to helping you stop the jabber and not want to screw up - it's not the be all and end all - but it's a running start
D