The very first thing to remember about being polyamorous (ESPECIALLY if you are just starting out, or looking into it) is that jealousy WILL happen. And, to me at least, it sound like that's what's happening here.
The difference between being poly and not, is the ability to talk through the jealousy/problems with your S.O.s and get passed them. If your SO has taken a step forward too quickly, explain to him, and the potential that you need to slow down and make sure that everything is working out. Assuming that the girl is mature and used to the poly lifestyle, then she should understand.
That's all I can really say. Because each relationship is unique to the people who are in it, the exact steps for fixing problems only you can come up with. However, as for being irrational, no, I don't think you are. Perhaps nervous and/or scared. But, being the fact that you are new to this, some bumps in the road are bound to happen.
Shrink and Rei
N. Little Rock • M/F Couple