@Chocosprinkle "I've been married to a lovely man for close to 15 years. I can't fault him".
I was also married to a lovely man and a fantastic father for over 15 yrs but , i was not happy. I started to become resentful and attracted to others . I did talk to my DH over the yrs but nothing changed. I divorced. I am much happier and freer now. My children are also fine and Dad is still a great Dad.
I am also currently very close to a work colleague exactly as you are and describe, he is married; i am not. He stopped talking to me about his wife a few months ago and i then noticed a change in our interactions, how he looks at me ( he looks at my mouth, my body, holds eye contact, he never used to do this) and then how physically close our bodies were when we are together at meetings, breaks etc, in the others personal space comfortably and physically touching. Then colleagues picked up on it, started incorrectly speculating. I have totally pulled back because he is married with a young child, so much so , we now currently no longer speak. He looks miserable and i am miserable. It has become very awkward for me at work and now him. I miss him and , were he were not married , i would love to be with him.
My point is , i am free to leave this behind and pursue new relationships and i am. Maybe you are feeling same and this is indicative of how you feel and where you are in your life and marriage.