Your deadline for a Motion for New Trial has long passed. I would contact a knowledgeable attorney and schedule a consult to see about filing for a Bill of Review. The attorney will need to review the Mediated Settlement Agreement and the Decree carefully to discuss your options.
It is possible that you can get it changed, although it may have been too long. Did you sign the Decree? When was the Decree signed by the Court? There are time limits for fixing things like this. You will probably need an attorney to get it done. Good luck.
It's way too late to modify the decree. It is final. There is likely language in the decree that it represents a merger of a mediated settlement agreement and that any differences between the two- the decree will control. I'm sorry. Lawyers are quite often essential because of the money potentially involved.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.