Answer:
When you are ready to tell her, then you need to tell her--it will not be easy, but it can be done. By being ready, I mean financially and emotionally ready to go out on your own, having friends and emotional support. Time will not change your orientation, but it will help you be comfortable with yourself. I think you recognize the possible/probable outcome of being frank with your family (not all of us are perfect), and probably the reaction will be the same whether you have the absolutely appropriate words and tone or not. There are so many more things that you can spend your time on, taking your sexuality in stride, that you may not have to have this discussion here and now.