My Fiancee wants Sex, how do I tell her I have an STD – Man Laments

My fiancee which I intend to get married to later this year wants sex from me badly but am scared to have sex with her because am having staphylococcus which have not been able to get cure for.
Yes, I contracted this because I cheated on her sometimes ago when we were far away.
We both work in different cities but she comes around sometimes when she is chanced at work. The last time she came, she was so Hot and all over me. I had to turn her down that am having headache and am not in the mood. I didn’t do anything with her throughout the period she spent then because I can’t afford to infect her.
She is here again for the first time this year. Left the room for her yesternight because she really wanted it. Just returned from work now and I saw her in a pensive mood. I love her so much and I can’t afford to infect her. Thinking of calling off the relationship but it may really be suicidal on her own part. She has introduced me to her family and my family knows her as well. Really, don’t know what to do. I can’t just have sex with her. She is a good girl than to start treating this seemingly incurable disease.