Hello Mark welcome here
I'm afraid all the solutions to your case/problem should come from your gf's decision,give her time to fix her problem (withdraw the communication and financial support from the father of her kids,they are not married and the kids are minor so the sole custody should be awarded to the mother)and take it from there...It will not take long if she is serious to move on with her life and relationship with you otherwise,its only you who becomes miserable and will be in great trouble in the future.
Goodluck and don't give up on hope.