I have very recently (past few weeks) found myself in a very similar boat.
For more than the past decade that my wife and I have been together I have struggled with my attraction to & feelings for other women (but luckily I succeeded in never acting on them in any significant way). In fact, I believed so much in these feelings that I encouraged my wife to believe in similar feelings too and encouraged her to act on MY beliefs hoping that it would open the door for me to practice them myself. Only recently, after having another bout of feelings of guilt and immorality, did I find the spectrum of ethical non-monogamy that exists all around me. This has completely changed my life for the better.
Luckily for me, throughout all of these years together I have unknowingly been following the steps outlined in the link that kdt26417 mentioned above.
Over the past couple of days I have been having some very important conversations with my wife about myself and our relationship. Due to the good communication skills that my wife taught me over all these years, I had previously expressed to my wife my personal ideologies on non-monogamy. Of course, at the time I didnt fully understand my own feelings and therefore neither did my wife. She understood and logically agreed with what I said to her (that my love for her could and would be unchanged if I were to ever have a relationship with another woman) but I think that our cultural programming simply rationalized, for both of us, what I was saying away into a sort of "wandering eye" mentality (for lack of a better term).
The fact that she understood my belief system was a huge boon for me. She understood it in the abstract and had been given plenty of time to come to her own understanding of what it meant for me and us although at the time it was all hypothetical.
Even though my wife knew about and understood my feelings the first conversation that I had with her was very difficult for me. I have not been a very emotionally expressive person in the past and when I first told my wife that I wanted to have an open marriage (I am not sure that I am actually polyamorous either, but time will tell I guess) I really struggled to say the words "I want an open marriage". I was so very much afraid of hurting her by just straight up telling her that is what I wanted. But the conversation actually went very well. She said that she kept waiting for the other shoe to drop because she didnt see what I was saying to her as real news. She was understanding and said that we should take it slowly (of course I agree) and continue to focus on each other.
From that first conversation we ended up in a second conversation where my wife told me that she wasnt sure she would ever be OK with an open marriage. This hurt, BUT, I refused to let it get me down. I have accepted who I am and that my personality is not inherently immoral or wrong. I cannot change who I am nor should I but that also doesnt mean (for me) that I want to give up what we have together in order to explore what I have buried for so long. Our conversation was interrupted by one of our children before we were able to finish.
The next (and most recent) conversation that we had about the topic was last night. It came about naturally (I didnt want to force the issue because I felt that continuing to bring up the topic would cause her to close down more on the idea) and we talked at great length about it. I managed to describe it to her in such a way that she finally realized what I realized several weeks ago. That what I was saying wasnt just hypothetical nor was it just "being a man" but that I had actually finally realized something about myself and my personality and my needs. In fact, she recognized why the first conversation had been so hard for me - I was basically "coming out" to her about my non-monogamous sexuality. She was very proud and happy for me and now feels like she understands me better. In fact, she is now on board to working with me on any issues she may have with my non-monogamy. I dont think I can really call it an open marriage because she says she has no interest in anything other than being monogamous with me.
I felt the need to register here and no longer be a lurker because of all the great stories, advice, and wisdom that I have found on these boards in the past few weeks. It has helped me more than anyone could possibly imagine. I wanted to give back and I hope this helps you even a little bit.
Only you will ever know where on the non-monogamous spectrum you fall and whether you are truly polyamorous (sounds like you are to me). And I have no idea whether the conversations you will have with your partner will make your relationship better or worse. But what I do know is that it IS possible and it IS important to have these conversations, no matter how hard they are and no matter what the outcome may be.
Open, honest communication and respect in a secure and fundamentally sound relationship. <-- The Key (as far as I am concerned at this point).
I wish you the best of luck!