Sorry to hear about your situation.
In my humble opinion, you need counselling. It should come from someone you both will listen to and put it into action.
It is also important to explain your husband that (the following is what I derived from the office of Sayyid Sistani):
1- Having a relation with a Na-mahram is haram let alone her being a non Muslim.
2- Turning a non-Muslim to become one in this case is either of the following:
a) because she wants to marry a Muslim- in this case there is no point in accepting Islam in this manner, and your husband should be informed of this.
b) she is wanting to be a Muslim after researching into Islam - in this case, it is not on your husband in particular to marry her.
Especially if the the second marriage will end up into spoiling his first wife's life and thier married life altogether.
3- I would also suggest that both of you see a local Aalim at your centre, that way your husband will be informed of all the rights that both wives have on him at the same time! It is undoubtedly quiet challenging and trying to be able to keep up with the rights of both.
On that note, I would ask you to pray for yourself and your married life, duas really seals the cracks when nothing else can.
You are in my prayers.