I agree with the others. You're going to be okay. You can open your conversation with, I'm scared and nervous.
It sounds to me like you may have a bit of classic anxiety going on that has nothing to do with polyamory. It sounds like you're "horribilizing" everything, making a picture in your mind of how badly it will go and getting upset about it. Some good techniques to deal with this are remember to breath and tell yourself that no matter what happens, you will all be okay. This isn't the beginning or end of the world, it's just the beginning of one conversation; a conversation that might take a while to have.
I'd try to look into some grounding techniques and keep on reminding yourself that you're going to be okay. Try not to get caught up in what "might" happen and instead remain grounded in what is happening in the present moment. Notice your emotions, but don't believe all the horrible things you're telling yourself could happen!
If you keep on getting panic attacks, it might be a good idea to see a counsellor for some suggestions on how to keep yourself grounded and begin this conversation with yoru wife.