How to tell her you like her without ruining your friendship?

Okay... So this is kinda complicated... This girl and I have been best friends for over two years. It started out, just friends, you know? I never really thought of her in any other way... Ever since... March maybe? I've started getting feelings for her... I've always been bi, and I didn't think that would get in the way until now... She's straight. She knows I'm bi and she's okay with that. I would usually ignore these feelings exept for the fact that lately, I can't get her off my mind. And I've been having horrible depression spells over all of this. The worst part is, I'll be spending the entire week next week with her at the beach. I just don't know what to do. I really want to tell her how I feel, but at the same time, I don't... What if it ruins our friendship? But then again... What if she decides she feels the same way? I just don't know what to do... Please help.