It’s easy to have a crush on someone because you get on well and have never had to face adversity together, heck you have probably never actually seen him in a bad mood. My point is knowing him at work is not the same as knowing him in a relationship.
Added to that, it’s very easy to get complacent in a long term relationship. All relationships have ups and downs, sometimes there is a phase we’re you feel more like friends than romantic partners.
You are not a monster for having a crush, you are human. Please take a step back and look at the reasons you are attracted to this man. Have a long, hard think about what may be lacking from your own life or relationship that makes this man so appealing. For example, maybe your existing relationship lacks spontaneity or you feel less desirable. It might be that the attraction you feel for this man is simply a desire for something you feel is missing.
Also, Google the term limmerence. Seeing your colleague gives you that dopamine hit, which makes a crush and any resulting affair like an addiction. Affairs very very very rarely result in a lasting relationship between the affair partners. It is far more likely that you will blow up your personal and work life simultaneously if you act on this crush. Plus when the excitement of a new relationship fades, maybe this man is just not as special as you think.
I recommend watching” Rece Affair Recovery” on tick tock. She gives practical suggestions on how to stop thinking about a crush. She talks in depth about limerence and the addictive feeling of having a crush.