Often times people take the decisions the people in their lives are making too personally.
When trying to determine how to feel about what someone in your life is doing or choosing - keep in mind a greater sense of justice.
Ask yourself some simple questions like...
Is this even about me?
Is it really up to me to judge this choice or feel any particular way about it?
What reaction or actions would serve the greater long term good for the people it affects the most?
How can I be magnanimous, generous, empathetic, mature and forgiving in this situation?
A good example is when an older child of divorced parents is faced with a parent who finds new love for the first time.
How should that (young) adult feel and act?
They would ideally recognize that, for the most part, it really has absolutely nothing to do with them and they should feel nothing but gratitude and happiness that their mother or father has found someone to give them tenderness and love in their life.