I'm in love with my best friend (both female, both bisexual)

The situation:

I went to highschool with this girl and always thought she had the most captivating personality. With her being older than me, we didn't hang out, but a similar interest in music and performing allowed us to work together a little bit and know each other. We went to separate universities, but got in touch a few years after high school and ended up being working musicians in different bands together. We instantly hit it off, moved to a city together and became almost like sisters. We tell each other EVERYTHING. We spend a lot of time together, since we live together, work together and perform together.
I came out as bisexual to her (she was already openly bi).
While being roommates, we had threesome scenarios with men happen and both seemed to really enjoy it, though that hasn't happened for over a year now. After each occurrence, we talked in depth about the details and whatnot without any awkwardness. We have both seen people off and on (her more so than I). We have never had solo sex.
Somewhere along the way, I gained feelings for her. It never seems like a good time to spill to her. People have always made jokes/ speculations about us being a couple, to which she's always reacting "no, its not like that".
I've asked her (while we were tipsy) if she'd ever wanted to just fool around with me and she said "no" passively.
I feel guilty about not telling her and it's tough for me to see her with people that don't appreciate her when I have nothing but genuine love for her. When she describes that ideal person for her, she literally describes ME.
I am pretty sure these feelings aren't mutual unfortunately.
I just tell her everything and feel like I need to tell her this so that I can get past it if she doesnt feel the same way.
I don't want to ruin our best friendship.... that's my ultimate fear. I am the person she most counts on and I don't want her to think I'm gone just because we don't feel the same about each other romantically.
The hardest part is, we are supposed to start an amazing job opportunity together, in which we will be staying in the same flat. I don't want to ruin this opportunity for either of us by telling her as I'm afraid she won't go anymore once she finds out. My good friends say that if we are really that close, she'll come around after getting over the initial shock of it. Im still terrified of telling her.
There is a lot on the line here. But how long can one go on feeling miserable?