Mary-
You're so not alone. I consider myself a pretty intelligient person and I had worked with teens as a counselour, so I know that self-injury is not the way to go. And yet, when my anxiety began at the beginning of this year (well, school year), I started hurting myself and making myself throw up (I think you Brits call it 'sick'...). Not some of my finer moments. And, like you, I knew that I wasn't going to do it again, because it was a bid for attention. I needed my family to know that I needed (and still need) help.
You have to know, that as bad as it feels (and trust me, I know it can feel pretty bad), it WILL get better. I'm already getting better (though I still have a ways to go)... best of luck!