Dear Troubled,
I hear your distress over your dilemma. There are no easy answers but allow me to suggest some factors to consider as you decide.
Firstly, you have not mentioned the role of faith in this process. If your relationship with God is important, the first person you should seek counsel from is the Lord. Has he been leading you in a particular direction?
Secondly, do not rush to make a decision. In general, it is wise not to make major decisions when one is in a state of emotional and mental distress. Take time to prayerfully and thoughtfully consider what to do.
Thirdly, your reasons for either staying or leaving stem from a negative motivation: that of a fear of triggering resentment in your children or yourself. Would you be able to reframe the situation to let your decision be driven by a positive motivation? This could be having the faith to do what you sense is right and / or out of love, such as unconditional love in action for your ex-husband who needs your care, or love for your children to shoulder this burden together with them. A decision that comes from a deep place of faith, love, even hope, will allow you to weather the inevitable difficulties that will come with whatever decision you make.
Finally, this is a complex situation involving many factors such as your relationship with your ex-husband, his relationship with his children, your financial situation, as well as your own difficult emotions. It would be best if you are able to seek wise counsel from mature brothers and sisters in Christ that you trust.