How can I get over being so frigid?

I love my bf but I can't kiss him cos I am so frigid. He wants to but I can't - I don't know why but I just get really embarrassed and scared. I have asked this question before and all I got was stuff like 'just do it' but it's not that easy:( I don't want to be frigid at all! But I am worried about having braces, or bad breath and that I will be a terrible kisser or will mess up!! :( don't know what to do cos I really want to kiss him! Scared he will get bored of me if I don't!:( help please? What can I do?? X btw never had my first kiss:(