You are not alone in being unable to question the premise that indissolubility is a rule made by Jesus.  I read an article in First Things earlier today in which the author tried to set out different and opposing views on chapter VIII of Amoris Laetitia, but he held to the same premise in describing both views.

It is a serious problem for the church.  Rules are about drawing lines.  Jesus was about something else.  St. Augustine put it concisely: "love God and do what you will."  The value of the pastoral focus of Pope Francis -- and he is not touching the rules -- is that it gets in touch with the real lives of people and turns the church toward a different way of teaching.  Instead of teaching rules, teach getting in touch with the God who loves us.  

If indissolubility is taken as a rule rather than an exhortation, there are too many palpable injustices in the real lives of real people.  That's the way rules are, which is why ascribing this as a rule of Jesus is idolatry.