You are living a difficult situation. She is a real friend for you and you fell in love with her. Sometimes, friendship can be ambiguous when you are very close of someone. She is the first person to be so important for you so it is normal that she is (and will stay) someone special for you.
I know it isn’t easy at all, but it can be beneficial for you to talk to her about your feelings. There is always a risk, of course. However, it’s the only way to really know how she feels. Moreover, it is your best friend; there are chances that she will understand you and accept it.
There are other possibilities, such as only talk to her about her sexual orientation. If you clarify her situation it can help you chose to tell her or not that you love her. As chosing this way, you are more sure to not destroy your friendship with her.
Whatever you do, it should be helpful to have someone you trust to talk about this situation and the emotions that it causes. It is easier when you aren’t alone with your problems.