Once you involve the police in a domestic dispute, the case does not belong to you. It belongs to the state and the prosecutor can handle it in whatever way he or she sees fit.
Even if you write to them and say you don't want to press charges, the state has the ability to go forward with the case, whether you like it or not. Although you said you wouldn't be cooperating, the fact is that if you are lawfully served with a subpoena, you can be forced to testify or to face contempt charges and a possible jail sentence yourself.
So in this case, though you may have had one set of expectations, nobody promised you that they were going to drop. The DA apparently feels he still has a case he can prove beyond a reasonable doubt, and the defendant would appear to have believed that too, because he took a plea.
There isn't much you can do now, except that if you don't want the defendant to be found guilty of these charges, you can contact his defense lawyer and tell him you don't understand what happened because you sent a written letter to the state prosecutor and the judge telling them you did not wish to pursue charges. Perhaps he may be able to get the plea back on those grounds if he wasn't made aware of this fact.