The best thing as has already been mentioned is marriage counseling. Otherwise it would be good to get one of his friends or family members who are close to him involved--he needs outside perspective. I realize there are two sides to every coin, but for someone to have made a commitment (even if I don't believe in "marriage" anymore personally, it's still a long-term contract) and to have had children with you, he owes it to you.
And as far as "letting go" of a woman he "loves," I'd ask him two things: if he really loves this other woman, "what will she always fear about him (and rightfully so) if he abandons a 25 year marriage and two kids?" Answer: that he will leave her. Then, ask him if he really loves her, he should see if the relationship is healthy/good by breaking a connection from her altogether for a month before going back to her. Not a commitment to you, but to himself and to her. To get perspective and a more realistic, less emotional foundation.
The only way I have been able to get unhealthy thoughts out of another woman out of my mind (unhealthy = unrealistic; I'd never entertain thoughts while I had a SO in the first place) was to completely cut her out of my mind for a while. Just got done doing that, but this would be easier for me as a) the woman had no feelings for me, and b) the friendship wasn't nearly as deep as your husband's is. Either way it's the only way I can see him staying.
Having said all this, it seems to me that your relationship with him is not healthy in the first place--I have gone to marriage counseling and read a few books about it myself, years back before I got married. One primary book that was recommended to me was called His Needs Her Needs and although I don't usually recommend it due to some religious and sexist points in it, the concept is very very accurate and helpful in my opinion. Check it out, and suggest he read it as well. It's a quick and easy read and helps understand what affairs are, how to avoid them, and how to maintain a healthy relationship.