Alas, polyamory is a great thing that can lead to even greater things, but usually starts out by inserting pure chaos into one's life. People frequently don't plan to "become poly" ahead of time, they just find themselves "falling in love with the wrong person" and trying to figure out how to navigate that without hurting those other persons in the equation who signed up for monogamy and monogamy only.
Everyone's going to have to make some major paradigm changes in how they see things and how they deal with things. It's going to be a rough, steep, surprise learning curve.
Earthquakes and upheavals will follow. Relationships will change. Relationships may end. All bets are off this early in the game. You don't want to be a replacement but she might have her own ideas about that. You might end up having to decide how you're going to cope with that, whether you're going to accept her plans for your life, and whether you're going to welcome her into your life (and home?) with open arms.
It's even possible she could swing the other way and conclude that to save her marriage she must jettison her relationship with you. So, pretty much, brace yourself for whatever you won't expect.
The only good news is that if the three of you survive this storm together, you could come out of it with something really beautiful on your hands. Just the dream is worth fighting for, I think.
Baby steps. Hang in there.