Somewhere along my poly path I've heard many, MANY times to go only as fast as the slowest person in the relationships.
This, to me, makes perfect sense. It gives ALL parties a chance to work through any problems they may be having and gives the person who is slower in accepting the situation MORE time to come to grips with what ever they're having a problem with.
From the get go my boyfriend & I have had the steadfast rule that we WILL close the relationship if there is something in our core relationship which needs fixing. We have done this twice. The first time was when I was having huge jealousy problems with his first relationship other than me. The second time was just after my Mom died so I could grieve without having other people who are precious to us being hit by the shrapnel.
This might be a good way for you to go. Set a reasonable time limit (say three months??? with the proviso that it can be extended if needed) so the two of you can work on your core relationship.
There is absolutely NOTHING which says this girl has to move in with the two of you (that's the way I read what you wrote, I apologize if I'm wrong).
I agree with what LovingRadiance said as well and will give a resounding second to http://www.xeromag.com
. I have used this site many times and recommended it to countless others.
Until you can get to your therapist just remember to breathe. Go outside and enjoy some nature in the form of the birds singing, the flowers blooming and the sun setting (or rising), or whatever it is you do to find peace within yourself. Find some peace within your soul. When you've had your 15 minutes of peace and quiet you will be better able to look at your situation more objectively and maybe see a solution, or partial solution, where before all you saw was a brick wall.