How can I stop fainting at blood?

For years I have had the problem of fainting around blood. Even if I read or hear about gory things I get woozy and p out. One time I was reading a book in school and it got really gory. I asked to go to the washroom because I couldn't stop thinking about it but it was too late and I ped out in cl. It was so embarring but I just can't help it!! Does anyone else have this problem, and how can I help to solve it?