I just read your intro and your situation is very similar to mine. I too worry about the pitfalls and the things that we might not see coming...but I think as long as you continue to communicate, things can work out. Is your GF feeling any better as far as her concerns about coming between you and your husband? Mine struggles with that, no matter how much reassurance we give her that she has a place with us.
I've started spending weekends at her house and I love love love it! I can't get enough time with her! It's amazing to be able to spend the whole night with her, and it's something I very much look forward to every week. I feel like the NRE is starting to wear off between her and I, but the weekends we have are still incredible.
When the 3 of us are together, I feel full of compersion and love...it's truly amazing. They get their alone time together as well (in fact our families are going to dinner tonight, then P is going over there for some time) and I'm enjoying seeing their relationship grow.
I have no advice for you other than to keep talking...and talking...and talking!
I'm looking forward to more posts from you...your story is definitely one I'll be watching, since it's close to mine!