Copenhagen

Good people of the forum I need your valued advice. I'm having a serious conflict of emotions and could use your experience and opinions. I've been with a long term partner who I thought I loved. We aren't married but are planning to do so in the next few years. I have however been increasingly attracted to another who I work with. We have known each other for a while and I feel the friendship is growing into something else. There are many signs that this is mutual. What should I do as no matter how hard I fight this I can't get my work colleague out of my mind?