Dark thoughts

hi I'm new here my child is 11 and awaiting assessment for high functioning/Asperger (referred 2 years ago). While she has always been a serious child she now has dark thoughts all the time. She talks about dieing and killing herself and constantly goes on about being no goid at anything and whats the point as she is useless. I have tried so hard telling her what she is good at, how much we love her etc etc but it's not making any difference I'd I don't know where to turn for help