My husband and I found ourselves in a similar situation. We tried fighting my feelings for several years. Finally we agreed to open our marriage to his best friend last October. It's had its grueling moments, and there were times we both thought we would divorce. But as of today, it's working out very well. Some of the initial emotional shock has worn off and we feel very stable in our marriage. I have had the chance to love someone who grew to mean so much to me, while keeping our marriage and our family intact. The fact that my husband loved me enough to let this be possible -- wow, it makes me love him all the more!
The beginning is H-A-R-D. There are so many things to work through -- setting boundaries, being brutally honest, it is ridiculously difficult. In my opinion it's been worth it, and I know my husband is happy we are still together. He still wishes I could have stayed monogamous, but he accepts the love I have in my heart for his friend. Bonus -- it has brought him and his friend closer too.
Watch "Butch Cassidy and the Sundance Kid." We've sort of modeled our whole V after the relationship in the movie!
Good luck, and stick with the forum -- there are people here who can help get you through anything.
Formerly married to Sundance
Boyfriend -- Butch Cassidy