She might feel close to you as like a friend. And that might be more special to her than the guys she just likes. It might not have occurred to her that you like her, in that way.
Tell her that you like your friendship. But that you also like her romantically.
You mention you'd be heartbroken if you went for it, and it didn't work out. But how would it be if you never went for it, and lost her because you never declared yourself?
Don't make it an ultimatum. Tell her how you feel, but that most importantly, you want your friendship and sense of enjoying one another's company to continue. But this way she has the choice.
And I think she'll like your honesty. Plus she'll see that you are not afraid to let her see your real self.
