The little girl is going to be in a world of pain when this ends. I really get bothered by that. Kids attach and when adults are sloopy with their relationships it's a the kids that suffer. Not to mentuion the adults who love them.
I think that if I were you I would find my own girl friend and stop sharing. It isn't necesasary and I have seen very few examples of fly by the seat of our pants whirl wind triads working out. This was way too fast to evolve on top of the less than positive out comes that start in this way. Triads seem better to evolve than be forced. Try a different approach and be independent. Co-dependent couples looking for unicorns create a lot of pain for their women I find. Might be something to think about. Your involvement is with three people not with a pet. It doesn't seem you realize that.
Next time: take your time, don't move her in until well after the NRE is over (like a year at least), find your own girlfriends, and don't take on other peoples kids until you are DAMNED sure you can be there for the long haul.
You might want to do a search in the tags for "moving in" "unicorns" "triads" and maybe "jealousy" there has been many threads that address what you are concrned about that might be helpful.
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