Ok, first off I wanna address something. Please don't use submissiveness as an excuse for cheating. You CAN control yourself, you CAN walk away, you are a rational being who is responsible for your own choices even if certain things really get you going. I mean, imagine if someone was REALLY into redheads, like instantly gets horny when a redhead winks at them, and said they couldn't help cheating because the person who came on to them was a redhead?? Sorry, no, you could indeed help it.
I don't say this to guilt you, since it's clear you feel a lot of guilt already, but it's no more valid to say that you couldn't keep yourself from cheating because you're a sub than it is to say that it wasn't really cheating because it was just oral. I think you know that neither thing is true at all. I say all this as a submissively-oriented person myself, because I don't think it's ok to use the way we feel and the way we love as a crutch like that.
Now that that's out of the way, a few things jump out at me.
- "just the thought of being without him frightens me to the core" That makes sense, you've been with him since you were basically little more than a child, for close to half your life. You don't know anything else. But if this relationship ends, you WILL be ok. There are other loves, other lives you can live. I know it's scary, but if you hold onto that terror, it will control you and your choices. No relationship should be based on fear. I think a good first step would be working on letting that go to the extent that you can.
- "Yesterday, we came near to kissing, though both he and I have been trying so very hard to tone things down. We want each other desperately." There is only one right thing to do right now. STOP HANGING OUT ALONE WITH HIM. This comes back to the idea that you CAN control your behavior. Take yourself out of this situation and you won't be able to cheat. It really is that simple. If you're on a diet but honestly can't keep your hands off the cake, put the dang cake out of sight, chuck it out if you absolutely have to (I hate saying maybe you should give up this friendship, but... how serious are you about not being a cheater, really?).
- "However, we both know that our current partners are the best matches for us, and we don't want to hurt or lose them." So, your partner is someone to whom you cannot talk about your real desires, sexually or emotionally. How is he the best match for you exactly? Wouldn't it be better to be with someone with whom you could be your real self? In the end, that's the ONLY way you can escape these constant conflicts, this depression... by being allowed to express who you truly are. Which, hey, might be someone who's kinky and poly.
So, right now, in this current context, I would say your partner is NOT your best match, if he's someone from whom you feel you have to hide yourself. That's just going to get harder and harder. However, he COULD be your best match... you've just never given him the chance to show you. By never being honest with him, you've never gotten to find out if maybe he could give you the things you want, or could allow you the freedom you crave.
There are a few scenarios going forward.
1) Don't change anything. Cheat with your friend.
1a) Your partner never finds out, but you feel terrible about yourself, as well you should, since you're betraying the person who's supposed to be closest to you.
1b) Your partner finds out. The fallout will be bad, very possibly relationship-endingly bad, and almost certainly much worse than if you'd told him yourself. You still feel terrible about yourself.
2) Stop hanging out with friends to whom you feel this overriding attraction. Don't cheat, since you don't have the opportunity.
2a) It works! You never tell your partner about your past infidelities, and the guilt fades over time.
2b) It works! You tell your partner about your past infidelities. From here there will very probably be a rocky period, but you guys should be able to get past it. If nothing else, you'll be able to have conversations about non-monogamy, and maybe see if open, ethical non-monogamy could be a possibility some day.
2c) It doesn't work, and you revert to option 1 above. :/
3) Tell your partner about your feelings for this guy. May or may not include telling him about the past infidelities as well (I would suggest that you get it all on the table at once, since it'll probably come up eventually anyway, and a succession of blows spread out over time teaches him to believe that he can always expect a new revelation/betrayal on the horizon, whereas telling him all of it at once lets your start with a clean slate).
3a) He doesn't take it well. Maybe you split up. Life goes on.
3b) You have some hard conversations, it's rough for a while, but he agrees to consent to an open relationship wherein you can see this guy. You get to be your whole self, explore your attractions to others, AND keep your partner. As unrealistically idyllic as this sounds, there are many stories of things actually going this way for people here on this site.
Best of luck, whichever path you choose. Just, choose consciously, take responsibility for your actions, no more of this "I can't help it" stuff, ok?
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.