"If something is bound to happen, it will" is a phrase that discourages proactivity.

As someone who's tried his luck with every best friend he's ever had, I can guarantee you that there will, at least temporarily, be negative strain on your friendship if the sentiment isn't mutual. This doesn't mean discourage it. On the contrary, mutual honesty and openness is the reason you're best friends to begin with, and not admitting your extreme feelings for her will feel counterintuitive.

Be honest with yourself what you really want from her before you do something that might make the both of you lose control of your friendship. It sounds to me like you're going to approach the situation with the goal of eventually stealing her from her boyfriend, right?