Haven't read whole thread but just wanted to add my 2 pence worth.
I was with my ex (dad of my 2 kids) for 8 years. We weren't unhappy, weren't that happy either. Both of us were pretty bored with eachother, didn't even have that much in common, looking back. our sex life was boring. once a week (if that) with the lights off - to quote a cliche ;)
After months of the relationship steadily getting worse, Ex said he was leaving. I thought he had someone else, but he wouldnt admit it initially. but (long story) I found out. I was furious, hurt, and devastated at first. BUT within just weeks, I realised I was happier without him as I came to accept what I had know for a long time - that the relationship had run its course.
Now we get on fairly amicably, I even (bizarrely, I guess!) get on with his girlfriend. And I can tell that they are absolutely head over heels, they are so right for eachother in a way that me and him never were for eachother. As for me, I am in no hurry to start dating again but the prospect is exciting and I know I have the chance to meet someone and experience falling absolutely head over heels and have a great relationship, not just a boring, stale one that has run its course.
I guess what I am saying (in a long winded way) is that if OP's marriage was a good one she would not have got in to this situation. The very fact that she is in love (or even thinks she is) with someone else, suggests blatantly to me that her present relationship is pretty dead. I believe that if you truly love someone - you don't fall in love (or lust) with someone else.