My Secret Nighttime Desire

I'm 34 and engaged, but every night before bed I secretly fantasize about my female patients from work. Tonight, as usual, it was Sarah, I’ve never been more attracted to anyone in my life. She’s just one of those nurses with that special combination of intelligence and grace that makes her irresistible. My fantasy goes further than usual tonight, imagining myself undressing her slowly, feeling the soft skin under my hands, her body naked and exposed beneath me as I enter her from behind, thrusting hard against her slick folds until we both climax in a sweaty, desperate embrace. This feels wrong, like there's something deeply unhealthy about this. Am I going too far with these fantasies? Is it normal to fantasize about your colleagues? Should I seek help or is this just part of my sexuality?