Like Rachelina, I am in almost the exact same situation...the only difference being that his current girlfriend is not the first occasion of cheating, and we've already been down that road. But this time around, it's all fairly fresh with me as well - I've only known for 3 weeks.
That being said, I have been supported in the most amazing way here. I've been given things to think about, in ways I hadn't approached them before, and have felt lifted and supported over a very rough 24 hours this past weekend.
And like Rachelina, here is a link to my original post: http://www.polyamory.com/forum/showthread.php?t=3953
At the end is another link to a new thread on the blog pages.
I completely understand the feelings you're going through - the sense of betrayal, of not being heard, of needing time to process. And like you, I've felt like I've been steamrolled and offered some hard choices. I have done a LOT of thinking and processing...and pretty much every bit of it is here.
What's been most helpful to me is that by coming here, I've finally felt that I am not alone, and that is valuable beyond measure.
Please know that you are not alone either. I wish I had words of wisdom for you...right now I'm walking this same path, and trying to figure it out for myself. Read and research, certainly - I have been doing that a LOT, and it's been insightful and helpful - but also try to leave some space to just be...to take things moment by moment, and to create a space for you to simply breathe.
Holding you in my heart...