I have had a symelar difficult dilemma, my thread titled "should I tell my wife" is where my story is, in short you are going to have to tell her.
"There isn't one. I've looked hard for one, and have spent a lot of time here and elsewhere. It boils down to this: use kind words and kind language, but DO share your feelings honestly. Maybe in the first conversation, stop at that--don't ask for anything, just tell your SO how you have been feeling, and how much it has been hurting to bottle it up (if you're anything like me, you don't have a choice in the matter; it's how you're wired: repeatedly falling in love and deliberately letting that love die of neglect HURTS, and that will happen to you over and over again for the rest of your life if you try to live as a mono), and how scared you have been (and of what).
Then STFU and let your SO talk. If your SO doesn't want to talk, then continue and perhaps say something like, "You don't have to say anything, and I'm not asking for any kind of a decision right now. I love you. There is nothing wrong with you--in fact, this is not about you at all; it's about who and what I am, and I need to be able to be honest about that. I do not want to leave you, and I will not act without your consent; however, I do need you to really consider this: read about it, think about it, talk with people and ask questions about it, and talk with me about it. Take your time."
I think this is great advice.
Even armed with this advice it ain't easy, think about it, mabe even make some notes about what you need to say, I find writing things out help clarify what is in my head.
Whatever the outcome is it will be better than not saying anything in the long run. Also the sooner the better, life is too short.