What you are sharing, although is not exactly what I am feeling and going through, it is somewhat parallel.
Do you believe that if you disconnect from her completely and she be involved with him on their own, that would help??....I guess what you can't see won't hurt you? Or do you find comfort in knowing and seeing?
You probably already know this, but you are his muse...find your inner woman, find you....find your happiness and bathe in it...(I need to follow my own advice
Have you established boundaries? Is it time to re-negotiate them?? Your fears and jealousy should not have to be tackled alone (I have learned here that your journey should not be your alone, you two should walk through it together) These forum members have taught me that you should go at the pace of the one that is having the hardest time with it.....
I hope I helped