For what it's worth, you sound extremely conflicted yourself. I'm not saying this to be judgmental, in fact I sympathize with the situation in which you find yourself, but I think that you really should pursue counseling or therapy as an individual, and couples therapy (with your wife). It is rare to be with someone that you have been with since the age of 15, so many experiences that other adults have had by this point in their lives are brand new to you & your wife.
But you sound very conflicted about this new girl in your lives. Many people say it, and I believe it - poly is not the cure for a bad relationship. And by your own admission, things were pretty bad between you & your wife when this girl came into your lives.
I think that you should both take a time-out from seeing this girl to work on your marriage. Be honest with her about why, so she doesn't blame herself, and so she knows that you both think it would be great if you could pick up where you left off at a later date, but for now, you need to work on your marriage. (And personally, I think you need to work on the conflicts within yourself as an individual as well.)
Good luck to you - this is a very heartfelt story and I truly hope it comes out best for all concerned.