Welcome to our forum.
Sounds like we have something in common in the way we started out with poly: I was also the "newcomer" in an already-established marriage. She and I got to know each other well and confessed feelings for one another. After that she did some research and found out about poly. She talked a lot with her husband and after about a year he was finally able to say yes to the poly idea. We became a V and have been together ever since.
I'm sure you will find yourself in good company here. Check out our various threads, and post more about your situation when you can. (If you put a link to it in this thread, I will follow it.
Glad to have you aboard,