You're so lucky he felt safe enough with you to tell you about his feelings for this co-worker.
Here's what I suggest:
Say to him, "H, thank you so much for having the courage to tell me of your feelings for co-worker. I know things haven't been great for us, but I am going to work on being the spouse I can be."
And then DO IT. Seriously, act AS IF your H is an honored guest (this really worked for me). Treat him the way you would treat someone who rarely comes to your home, but who you love and respect very much. It will feel weird and unnatural at first, but eventually, it will start affecting you and you will start remembering the man you fell in love with. And he will start remembering the woman he fell in love with.
Also, I would suggest you check out http://www.marriagefitness.com. Do the tele bootcamp with your H. It can be amazingly transformative. At the very least, sign up for the email newsletter. If you're really ready to make deep, lasting changes in your life and your relationship, you'll have to work very hard, but it will be very worth it.
Good luck to you!
ntl
Me: 30 H: 32 Dating 10/96 Married 8/01 H PA's: Summer 97, 12/06, 5/07-10/23/07 My Saga