I have gone back to my wife and will continue to live in a loveless marriage for sake of kids and for financial reasons. But I would give everything up for her.
sounds like you would benefit from some counselling. on your own, with your wife and with your entire family.
Your wife needs the chance to be with someone who loves her and it appears that is not you. Work on ending the marriage in a good way. You owe that much to your wife and children. The children don't deserve to see either of their parents tolerating unhappy relationships. They can, and should, learn that there are better options.
Sort out your primary relationship before you try to be in a relationship with someone as scattered as your former/girlfriend. Neither of you appears to be in a good place to have a good, supportive relationship.
Long ago I heard someone say that the best relationship is one where both partners are putting in 150%, so that if there is a gap on one side on a bad day/week, there is still enough to hold it together.
Your ex/girlfriend certainly isn't giving 150% to any relationship, and neither are you.
Sort out/end the marriage and then see where you are.
Be honest with both of the women. Then (temporarily - at least 3 - 6 months ) put a full block on your ex/gf.