Welcome Hopeful I hope this forum gives you the advice and or support you need. To answer your question NRE is usually applied to a new relationship, not the current one. Its when you start a new relationship and the new partner kind of makes you feel like a teenager again, giddy, blissfully in love, and excited while your new relationship grows, but if you've got these feelings from your current relationship then great for you and NRE fits well.
If "B" isn't up for being one of two men in your life then that will have to be something he thinks about. If he decides that he can't do it, even though it will be heart breaking for you, try to remember what you said in your original post, that there are many partners out there who can help you find happiness. I would suggest that if he is up for it, taking things in slow steps so that he (and all of you for that matter) can make the transition with out regret or hurt.
Good luck and welcome again