Under optimal conditions the MC should not be done if you are not having at least one unassisted poop per day pre MC. If that is the case for you then you should consider looking into colon cleansing first. Have you considered colonics? or maybe one of the easier to do oxygen based protocols like Oxypowder?. Once you are having one unassisted poop each day then you can do the MC to clinch the deal.
What you really need to consider is that barring any congenital or actual damage from years of abuse that your current lifestyle/diet is causing this constipation. The cleanses will help you take the trash out but that is only a temporary solution. You are doing a really good thing and pointing yourself in the right direction with the green juices and fruit but obviously that is not enough. After you do some colon cleansing fasting etc you need to consider what else you are putting into your system that may be causing this constipation and do some research to find something you resonate with and look into some overall dietary changes