Writing things down, here or to Jen, have definitely helped me. Talking through everything has gotten me to the basis of my fears. For me, it seems codependancy is what is driving my fear of poly. Not that I'm saying that is what is going on with you....
Just keep talking, figure out what it is that makes you not accept it. I initially felt the same way oddly enough. I didn't really have a problem of FWB. But after a couple weeks, that wasn't what I wanted at all. I wanted Jen to have that emotional connection. But then I started fearing that as well.
I have a little notepad thing on my phone. Lately I've been jotting things down as they pop into my head. I have a bad memory, so if I write it down, it'll help me to remember what it was. So writing things down for me has helped me a lot.
As far as getting another girlfriend... It's weird. It's something I would kind of like to do, since I think it would help me to understand where Jen is coming from. But at the same time, I don't really want to divide my time. So I think for now having a secondary relationship for me is pretty much out of the books. Maybe sometime down the line when I am more in control of my codependancy... But I'm not in a rush to find another, since I'm perfectly happy with Jen.