Be honest with her and tell her that you’ve been feeling more than just friendship feelings for her but you’ve been hesitant to say anything or to act on them because you weren’t sure how she felt and you like your friendship so much, you didn’t want to say anything that might change it. But the reason you’re saying something now is because the feelings haven’t gone away – they’re only getting stronger and you know if you didn’t say something, you’d always wonder ‘what if?’.
Here’s the thing: you admitting your true feelings to your friend *might* change the friendship, but you not admitting your feelings to her is bound to change the friendship as well. Friendships evolve and change – through circumstances and feelings. Your friendship right now is running along under a pretense – the pretense being that you are strictly platonic friends. But your feelings are not strictly platonic, and that makes the friendship less authentic. At some point – sooner than you probably realize – the inauthenticity of your relationship is going to change how the friendship functions. You probably already suspect that possibility and that’s why you’re feeling more and more like you want to say something – like you want to have some control in when and how the friendship changes.
So, take control. Tell your friend what your true feelings are and know that if/when your friendship changes – and it may change in positive ways! – it’s changing because it’s becoming more authentic. Authenticity may not always get you exactly what you want right away; but it will always serve you longterm needs better than hiding your true feelings.