Im in love with my ex husbands best friend

wow not been ion here for two years as took a complete break to work om my self.
I got divorced in 2012 due to bad parenting and an obnoxious stepdaughter we were married for 12 years ex has had a girlfriend for about 6 years now. His best friend also married with 3 kids so for 10 years we all hung out together, his best friends wife started to have affairs and really changed in to a diffrent person, she was my best friend but when she started to change and hang out chating up guys in bars , I told my husband that I dongt enjoy going out with her no more so stopped. She had an afair with a close friend of all of ours, so he would come and live with us,then she would take him back only for it to happen again. they got back together and remarried only for her to start having affairs again, so they divorced again. I had not seen him in 8 years and we got together for a catch up as we had been great friends. One thing led to another and now its 4 months and we are seeing each other in secret. We know each other inside out and we love each other. the problem is he sees my ex who is his ultimate best friend once a week. he is consumed with guilt. he wants to tell him but we just dont know what the outcome would be. MY bf is very careing and sensetive. we are in our late 40s. we have such a laugh together and love each others company and we are made for each other. we have decided a few times that we would stop. but within a few days we miss each other. I laUGH WITH HIM AND HAVE FUN AND IF THIS WAS TO be for the rest of my life I couldnt be happier. He has made me more confident in my self, I just dont know what to do,