Onoma, you are on the right track. Keep communicating with your girlfriend and make sure you both understand each other's wants, needs, and limits in the polyship. Get all your buckets in a row and keep a jar of spoons within easy reach all the time. Have her read "morethantwo.com", "The Ethical Slut",, "Opening up" by Tristan Taormino, and "Sex at Dawn". Join OK Cupid. See if there are any poly groups in your area. Tell her that just because people have a second kid, they don't love their first kid any less,, and poly is the same way except with romantic relationships. If that doesn't work beccause she is not a mother yet and can't possibly understand how mothers feel about their children, then try the food comparison. "JUst because I love pizza, doesn't mean I love ice-cream any less. So you could be the pizza and if i got another girlfriend, she would be the ice-cream. See?"
Try all of these things, and if nothing helps, let me know and I'll see what else I can dig out of the upholstery around here for you to try.
Oh, and remember - communicate,, communicate, COMMUNICATE!