Are you saying that we cannot trust the Lord to honor a covenant that is a legal agreement that cannot be broken? A testament, like a will and testament can have a new testament written and it is the one to go by, but a covenant always is in effect. The new covenant was superior to the old one, it made some of the old covenant not needed any more because the terms were better, but if God can cancel a promise to us we are in deep deep trouble.
The basic premise of the Mosaic Covenant is that when we obey law our life is blessed, and when we don't our life isn't blessed at all. We see that lived out, if we are liars, thieves, haters of others, we have a hard life and if we are kind and considerate of others our life works out easier.
The basic premise of the new covenant is the through the blood of Christ our sins are forgiven and we have salvation. There are other improvements the new covenant brings, too.