Can I sue my EX wife for Breach of Contract

There are a number of questions here.  First, most jurisdictions require more formality in these types of agreements (post-nuptial) than in arms' length agreements.  For example, some states require that they be witnessed and/or acknowledged, like a deed.  I don't know what Florida's requirements are, but the agreement you're relying on may not comply.  Second, lealving aside the formalities, an agreement creating a monetary obligation depending on marital fault would probably violate public policy in some states.  Again, I don't know what Florida law provides.  Finally, and most problematic, you should have sought this money in your divorce action, when the Court was adjudicating the financial issues between you.  Having failed to do so and the divorce action having concluded, you may be precluded from raising the issue now.  None of this is to say that you can't possibly overcome thise hurdles, but I don't think it will be easy.