I would say first you should contact a specialist lawyer in the states that can help you with the legalities of the situation and also help you locate the paper you and your husband signed. If anything it would at least be proof that at one time your husband wanted the divorce and it'll be invaluable if he keeps giving you the run around.
I myself know little about foreign divorce policies but according to this site (
Germany Divorce, Prenuptial, Separation, International Family Law ) the divorce might only be recognized in the US or in Germany but not both.
"The dissolution of a marriage is thus basically only valid in the state in which it was dissolved ... and so separate proceedings are usually required for its recognition elsewhere."
It also states that if you are decreed divorced in the US then you will be able to marry but if you were to ever return to Germany as a citizen it could cause legal problems with the marriage being considered bigamous. I'm sure though that you want the marriage ended on all grounds so a lawyer who can help you through the German legal system would be of great importance.
I'm sorry I can't be of more help but I wish you the best. Keep us posted!