I, too, could have written it myself. And I went through something similar, the past two months. My partner and I met a girl I was really excited about, who was really excited about us. We're queer women, so lines get a little blurry. I'm femme, the new girl was femme, and my partner is butchy. We are all most familiar and comfortable dating the opposite: I date butch girls, my partner dates femmes, the new femme dates butch girls. But me and this femme were into each other, and she was super patient and respectful of our relationship, so I was psyched!!
But I wasn't ready. And they wanted to also see each other without me. My partner is just like your bf: totally cool with me going on dates, sleeping with other butch girls. But this would be the first time either of us had dated anyone solo, and I was having a HUGELY hard time handling it. She and I have seen each other hook up with other people, always present, and she always says I can date people solo if I want, but so far, I haven't.
I don't think I can tell you that me or my partner handled it properly. We both made mistakes, and we have a lot of healing to do. What ended up happening was, I held on too tight, my partner got impatient, and cheated. Then I went behind my partner's back let the third party know what happened, and how I was feeling about it, and she was overwhelmed, and maybe a little irritated to be a part of it all. So she dipped out. I wish I could tell you we made it work and everyone was happy, but we didn't.
NOT TO SCARE YOU, because from what you've written, your boyfriend has been accepting of your boundaries and your struggle, and he deserves a lot of backrubs. That would be the difference between your experience and mine. Appreciate his efforts and don't let them go without praise, and hers too, for being patient with you. It's good you put off your relationships to ease up on your guilt about having your cake and eating it too. It shows you're committed to him and to working out the kinks.
One thing my poly friends suggested, that we didn't have time to try before everything exploded, was to give each boundary a time limit. That way, there is a light at the end. So say you don't want them to sleep together for the next... two months. But maybe they can do something, like makeout, or oral but with protection, after one month.
And see how each step feels, and be honest to yourself and the partners. A lot of the forums say go at the pace of the person who is struggling the most. So I agree with snowmelt, forget about fairness. If this becomes a habit forever, then you're being a hypocrite. But right now, you need time to adjust.
I don't agree that fears like these, which are perfectly natural, mean we need to reconsider our status as poly, especially when it's a new experience like this is, for you. You are poly, you are struggling, and no one can take away that truth about your sexuality just cuz it's hard for you now. If it continues to be hard after say, 4 months, then reconsider. But use this time to figure out what makes you tick, and rely on both of them to help reassure you that you are not being replaced.
I also think that time apart for the two of them may be in the cards, assuming they're willing. If they can pace their relationship slower, you may feel more calm.
My two cents.