So if her fwb is her happy drug? Does that mean quit cold turkey? I've had an emotional affair on my notsoDH. I don't want to tell him, he already knows the marriage is in trouble. What I am doing is trying my best for as long as I can stand it to fix what is going on. Talking to my DH about our issues, trying to go out both by ourselves and with other couple friends on activities, trying to bring back what I felt for him. I've left it up to him to get therapy for both of us. Part of my issue is it doesn't feel like he's trying even if he says he doesn't want us to separate.
Even if you can't do therapy there are some good books out there. I'm trying to get him to read a chapter while I'm out of town for work, then I read it and then we talk about it. (so far he hasn't and I'm 3 chapters in.)
Good luck, and remember... Life goes on and every day you have a chance to make a change for the better.