Being a third doesn't have to be inevitable
I've been where you are, and have felt the same feelings. But here is what I've learned. If I feel like the third, it's because I put myself there.
My partners have been married for almost 30 years. There is a dynamic between them that I'm not apart of. It might be a look, or a validation to a past event, or something as simple as finishing each others sentence. I can't change that, but neither can they. If we dwell on what we can't change, we don't pay attention to what we can.
Does it hurt when there is silence. Absolutely. Do you feel like it might have been the wrong decision for you? Absolutely. It is worth fighting for to find out? Oh Absolutely.
I've lived through the silence. I've also lived through the anger when I refused to accept silence. It is a new relationship. Everyone is trying to figure it out. The unfortunate part is because they have a longstanding relationship, we think they are supposed to have it all worked out. It rarely works that way. Everyone is unique. Our feelings are not the same, the way we react is not the same.
The hurt is real. But if you are truly a part of this relationship. Then you voice your opinion. You can't wait for someone else to make the decisions for you. You have to be a part of the decisions.
Let your g/f have time to collect her thoughts. Respect her right to get it together, but no one should control your actions or feelings. You alone are responsible for that.
I hope it works out for you. I really hope you all figure out a way to make it the best experience you've ever had.