Wow, sounds like you've been on a real rollercoaster over the past little while. It also sounds like you feel as though you've had a taste of something wonderful. That's a tricky place to be.
This "Big Love" is something that I share with my husband - it's the kind of thing you see in movies and read about in books, and when it happens to you it's like you've been let in on life's best secret. High fives for finding that!!!
I hear you not wanting to put that big love in jeopardy, but also that you've enjoyed your new connection and don't want to hurt this new young woman. The biggest and only real piece of advice that I'm going to put forward is - take the urgency out of the equation and be honest with all parties. You don't need to figure it out all at once, but it is important that you relay that you need time/space to work on your primary relationship to your new partner. "This has opened up a lot of questions and concerns in our relationship, and it's important to me that I respect my partner and you by taking some space" or whatevs.... honest, respectful, and giving you the necessary time and space to communicate and work out what you need both on your own and in your primary partnership.
Rushing has been one of my biggest personal pitfalls - when I don't rush, I make clear decisions that are best for me - when I rush and act from an emotional place it almost never goes well.....
Best of luck - keep posting and communicating!!!