Rider, I actually read RBR's thread about your freak outs before replying here. Hi there.
Looking at that thread and this one together, yep, RBR might be best served by reconsidering her weekend plans with E. A month is pretty new. It would be new for a mono relationship. It's new for a poly relationship. And it's especially new for a just-out poly realtionship. We all struggle with reeling in the urge to go go go, but experience teaches us that sometimes baby shouldn't run.
You might also consider blessing their outing. If they're really set on it, sometimes the best thing to do is let it go. Sucks but true. (RBR has already been collectively nagged for selfishness, and I think we'd all hope that she reconsiders.)
As for the two of you and adjusting to poly life, love may not be diminish with a poly person, but there are finite hours in the day. It sounds like you, RBR, and E need to understand that and work on it. As the hinge, RBR is most responsible, but it is up to all three of you to balance.
You personally might find that getting to know more about E and having some time together with the three of you will help dispell the boogeyman. The more human a metamor becomes, the less scary they are. I'd suggest to RBR that she keep that in mind as she works on regulating her pacing.
As a final note: "How do poly people do this successfully???"
Easy... we fuck it all up. And then we try again and while aiming for fucking only 95% of it up. And then we try again aiming for 90% fuck-up. Success is a goal, but it's never a destination.