Focus on what is right for the child first and foremost, then you. GF seems very unstable and there is no way you should marry her as things stand. You should write things down if there ever comes a time that you must fight for custody or even feel the need to pursue 100% full custody of this unborn child. What is her mental history and childhood like?
How exactly you should move on is up to you. It is a very special situation and I dont know the details well enough. Consider your values and all you have learned about people in the past few years and make a LOGICAL decision. What do you need to take care of yourself and ALL of your children in the next 12-18 months? You certainly dont NEED a GF, but your children all need a caring father AND a mother (given they are both mentally fit of course).
Me: 34 Stbxw: 30 D:5 D:3 Mini bd: May/June 2019 Married: Aug 2019 BD: 6th Dec 2019 OM Confirmed: Feb 2020 March 2020: I filed for D Waiting for D to be finalized and W to move out end of January 2021