I too have such a difficult time with H having FF. My father cheated on my mother with a so called "friend"/neighbor, he divorced my mom and then married the OW. I was 8 yrs. old.
I have always been very jealous of other women being around "my man." H knew how crazy I was about the issue, so I never had to deal with it until the "bomb." I am still not sure if FF/business partner/OW was ever anything more than just a friend. I think my H can be pretty niave. H is not a flirt by any means. I like to think that FF is just a very good friend of my H. I don't like it, but the more I say, the worse it is. You know what I mean, I'm sure.
So, what is the answer? I still struggle with my insecurities. I pray alot. I ask God to banish FF from H. God knows what H is doing, not me. I try to remind myself it is not for me to worry about. It is hard. What has helped me is the fact that we are S. If OW/FF called all the time like she use to, I would go crazy, but of course, I would have to act like it doesn't bother me.
Any advice on this issue, would be appreciated by moi too!! nik