Very glad you are wanting to tell her as you both deserve to have a life with someone they trust and share all of them self with. I would also not tell her until the semester break, but please no games or hints ect just say the truth. I would tell her you never shared this with anyone EVER and feel you can share this part of you. Stress to her you are the same person she has always loved NOTHING HAS CHANGED. Stress to her it is a common thing and you can get material for her to read AND do not make promises you will not be able to keep.
She may need time to process it and there might be a few ups and downs but you both will get through it My wish for you is that you two can explore this together