Having read further I wouldn't change what I have written; too fast/confident in results, niggling feeling got bigger and now has become jealousy... all what I said before. I like that you are going to spend some quality time with your first love this weekend. I hope that helps... maybe you will get some talking in about boundaries so that she has quality time with you more often.
I think I would ask her what it is that she is not getting out of your arrangement and then attempt to create some boundaries between all of you so as to better address the issues she has and perhaps what comes up for the other two of you... you never know what a deep discussion will bring up.
All this is part of a more mature relationship dynamic in that you are all settling in a bit more after the NRE is wearing off. Good luck.
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