First of all, I am very sorry for the situation you are in and the dilemma you face. Some of your feelings are very normal. The only way I can respond is to tell you what I would do (as I am separated) - I would make a decision on what part/role I wanted to play in this scene and stick to it as long as I can.
It is a terrible situation, but it is a dilemma not a problem to be solved. Problems have solutions, dilemma have alternate outcomes. Choose the outcome you can live with and take the steps to get you there.