Can't Stop Thinking About Her Anal Sex Fantasies

I met my partner when we were both grad students, and since then it's been a steady relationship. We've tried everything and have always found ways to make things work, but there's one thing that keeps me up at night, anal sex fantasies involving her. The thought of being inside her like that turns me on so much more than anything else we've done. Last week, I finally told her about it during a quiet moment alone in the bedroom. She was shocked and I could see fear flash in her eyes. We haven't talked about it since then, and now I'm worried I ruined everything. Is this normal? What should I do?