How do I do It and when do I do it? HELP!

I don't know if whether coming out to my bestfriend on my birthday is the right time

I mean, I've met so many people on Twitter/Facebook that have been through my circumstances and encourage me to come out whenever I feel comfortable. I promised my friend that I would be honest more about myself and such to her as a new years resolution and told her I would tell her some shocking news in April. A few months ago, one of our friends came out to the whole school well nearly the whole school (he was bisexual) but it was more of an attention-seeking act to the extent of our knowledge. Long story short, my bestfriend accepted him and as did I but she also thinks it was an attention-seeking act. Now I'm gay but I feel I would be more comfortable telling her I was Bi than gay because I do have feelings for girl and can only ever truly love a girl but I think I want to tell her I'm Bi rather than gay so she doesn't stereotype me but I don't want her thinking that I'm the same as this guy and just looking for attention. My birthday is next week and I plan on telling her but I don't know how she will take it because I'm just so paranoid, I've gotten so close to her lately and I feel paranoid that she may bitch about me BUT I DONT WANT TO SHOW ANYONE I'M PARANOID! I just don't know what to do, I want to tell her because shes the only one I need in my life but I feel like I'm going to breakdown when I tell her and I don't know what is the best possible way. Like say I text her and months later we are not friends, she could screencapture that text and tell everyone?