MJ, if your BF is really your best friend, she will find it in her heart to be happy for you no matter her own struggles. It may feel bittersweet to her, but that's her.
I would say, share the news with her when she seems to be in a good mood. If the conversation goes there, let her know that you understand her frustrations, and that you are supportive of her, but that you also need her to be happy for you. If you speak from the heart and share your own excitement, but share it with kindness, gentleness, and understanding - then you have done what you can do. She may need some time, but best friends usually come around!
I would say, share the news with her when she seems to be in a good mood. If the conversation goes there, let her know that you understand her frustrations, and that you are supportive of her, but that you also need her to be happy for you. If you speak from the heart and share your own excitement, but share it with kindness, gentleness, and understanding - then you have done what you can do. She may need some time, but best friends usually come around!