Oh dear. What a mess
From your description of things, it sounds as though they haven't put very much effort into making you feel comfortable with things, and that discussions seems to have degenerated quite a bit.
He agreed to be faithful to you and wasn't. Doesn't matter if he's poly, that's wrong. Now, he is asking you to consider a poly arrangement. What's really missing from your story is whether you want this or not...?
If you do, there's a lot of work ahead of you, and it begins with lots of research and honest, open conversation with all three of you. If you don't, well
I would just take things slow for the time being. Pull back a little and let your head clear. After all your strong emotions have cleared, you can take a more thorough look at the options, and act accordingly. If they are both interested in having you involved, let them know they need to support you, that your bf needs to acknowledge his infidelity and you all three need to realise that this may actually not be something that you want to or can do!
Not wanting to talk is a bad sign. I would say you're in an emergency situation
Let your bf know you love him, let the girl know you have nothing against her, and spend some time asking yourself what you want, what you need, and what you are prepared to give. Then, some honest communication...
I'm sorry things have been stressful for you... Stay strong