Thinking about another man

My fiance and got together in a bitbof a whirlwind relationship. I was not attracted to him straight away, it was something that happened in time. Before I met him this guy who works in the same mall as me kept on catching my eye. There had always been a strong chemistry and it was obvious that we both felt it. I didn't see him for ages and everything was going well with my fiancee until recently. I feel like me and my fiancee are drifting apart. And now at the time I need to keep my head I ny relationship. I saw this guy again at a party and have bumped into him a few times since. I can't get him out of my head, I become shy when I see him and try and not look him in the eye. But worst of all when my fiancee were making love I was picturing him. I've had two dreams about him. I just want it to stop so that I can concentrate on my relationship any advice ?