OP, it sounds like you're looking for a recipe for how to come out to your SO in some kind of a gentle way that will absolutely, positively, prevent your SO from leaving.
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."
If there is nothing happening for a couple of weeks, gently prod again. Offer to send a link or two (e.g., morethantwo.com), or read a book together (e.g., Ethical Slut). If you get an outright dismissal or an ultimatum, then you have a hard choice to make. You can take the chickenshit approach that I have followed for the past ten months--of cramming your poly self into a mono-shaped slot and being too scared to talk (which does not work and results in sleepless nights, panic attacks, and an inner feeling of betrayal whenever you look at your SO and feel like you're going to shatter her happiness when you finally have the conversation), or you can tell your beloved SO the truth: you are unhappy, and you can see that she would be unhappy if you lived poly, so you have to go your separate ways and find your own happiness. Tell her you support her whatever she decides (this is especially important if she is financially dependent upon you), and that you will always love her and will always be there for her, even if you are no longer together.
You have to be resigned to the very real possibility that this discussion could unwind your relationship. There is no way to have this discussion without taking that risk. There just isn't.
Last edited by MorningTwilight; 04-20-2012 at 07:40 AM.