Good luck Pisceanman. You are in an incredibly difficult situation with duty, loyalty, family and love on one side and waning NRE with the strong potential to develop into a monogamous love that you feel may be more fulfilling on the other side.
I hope these forums help you gather your thoughts. Your thoughts are complex and I suspect a trained psychologist may be able to help you better.
My personal belief is that the choice to have multiple partners must be icing on the cake of your existing relationship. If the existing relationship has a few problems, then polyamory is unlikely to truly fix these problems. Choosing to add multiple partners when the existing relationship is strong can add a lot of excitement but can also destroy an existing relationship that was not that strong to begin with.
My wife and I interpret this as core needs that have to be provided by the primary relationship. We're currently monogamous with the possibility to start polyamory in the future. If my wife feelsa core need to be heard and understood whilst I am unable to provide this, but instead finds a new man who can give this to her, then our primary relationship is going to be in jeopardy. If my wife has a desire to listen to loud music and dance the night away while I have no interest in such an activity, then polyamory permits her to enjoy this as icing on the cake of an existing happy relationship.
You said that personal development and a quest for knowledge and wisdom were important to you. You say that you feel your ex GF encourages you to achieve this whilst your wife has failed to do so in the past. If you identify this as a core need, I feel it would help your situation for your wife to acknowledge it as such. If this is merely icing on the cake, then it is less threatening. When I hear your wife responding with "shouldn't the kids and I be enough for you", I feel that she is hurt and threatened by what your ex GF can provide you that she can't. I feel you and your wife may benefit from a discussion of core needs vs icing on the cake.
This will have to be done diplomatically. Comparing one lover to another can be tricky. I would suggest phrasing personal development as something that you feel is important, an acknowledgement of her efforts in the past including the sacrifices she has had to make for you to achieve this goal, followed by restating that despite these setbacks you still feel a need to pursue the goal. State without emphasising it that you feel your ex GF encourages or allows you to explore these and forms a large part of your attraction to her. Look towards your wife with love and tell her you would like to try these things with her. The aim of this is so your wife feels less threatened by the GF and gives her a way to meet your core needs.
This helped my wife and I reconnect. I hope it helps you too.