What do i do?

Alright first time posting but am in a very bad spot and need help. I am an engaged male 24 years old, I have one daughter with my fiancé and one on the way. I love my fiancé but have almost no sexual feelings for her at all. Over the years we have been together (4 years) she has really let herself go. I am what i would call attractive and physically fit. My fiancé has helped me so much from day one taking a broken and broke man and turned me into a responsible one. I still rely on her for the majority of our bills and expenses. She comes from a wealthy family that I adore and they adore me. Recently I had an opportunity to engage in sexual activity with a coworker who happens to be the most beautiful girl I have known. Both personality and looks and she has a killer athletic body. So I took it, I spent the night with her, had sex and even cuddled all night. It was a dream, now i can't get her off my mind. Shes all I think about all day and just look forward to days that we work together so I can see her more. She just felt so right in my arms like no one, including my fiancé has ever felt. I should also mention she has a boyfriend. Now I know I am a terrible person for doing what I have done, but what do I do now? My daughter is my life and could never imagine a split marriage in her future or my new daughter due very soon. If I stay and marry my fiancé my life is set up to be pretty good both financially and hassle free. If I one day decide it's not going to work I have many struggles and very hurt family members in the path. WHAT DO I DO? Let me mention in less than a month her dad would like me to start working for his company and in less than a year he will spend over $70,000 on our destination wedding.

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