I agree with what others said about NRE, new relationship energy, being the culprit here. Your primary had her own dose of it, and now it's worn off and her jealousy has finally kicked in. Jealousy can be dealt with tho, it's very difficult at first, but gets better.
But I want to address this aspect which hasnt been talked about yet:
With this new girl I've started to discover myself sexually, and have found that I have very strong desires -- needs even -- that my old love simply will not fulfill. ...With my new girl I would be happy, sexually satisfied, and I would get the pleasure of exploration and adventure both sexually and romantically that I've never had the opportunity to experience.... It also feels like if I chose her, and her more exciting nature, her more sexual nature, her more adventurous spirit, that I would be choosing THAT over the powerful feelings I have for the first girl.
So, you're discreet about the details here, but I'd place a bet this is part of the problem. Since you and both your "girls" (can we at least have initials here to differentiate and humanize them?) have had 3way sex, your primary mustve witnessed the more adventurous (kinky?) stuff you and the new girl have done. Maybe this is a rather big part of her new insecurity as well.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
The single biggest problem with communication is the illusion that it has taken place. --Shaw
me: Mags, female, pansexual, poly, 59, loving and living with
miss pixi, female, pansexual, poly, 37