This is an interesting question, and you really will need to have a lawyer spend a little time and research this for you.

Ordinarily, agreements say that alimony will stop on remarriage, and if the document says nothing, the law says the same thing.  So, what I don't know is whether your agreement, to continue paying alimony after remarriage, is enforceable.  It might well be, but it might also be considered against the public policy of the state and therefore not enforceable.

If it's not enforceable, whether or not you could recover any of the money paid since her remarriage is another difficult question.