Exemptions go to the custodial parent unless there is a court approved agreement or order that says differently.
My name is Stephen R. Cohen and have practiced since 1974. I practice in Los Angeles and Orange County, CA. These answers do not create an attorney client relationship. My answers may offend I believe in telling the truth, I use common sense as well as the law. Other state's laws may differ.. There are a lot of really good attorneys on this site, I will do limited appearances which are preparation of court documents it is , less expensive. However generally I believe an attorney is better than none.
Your logic is faulty. You claim you were not given the 8332 authorization for 2011 or 2012 so you cannot now afford your child support. Prior credit does not give your present ability to pay. If your child support is too high, go back to court and seek a modification. Furthermore, unless there is something in your Judgment for Dissolution, your ex as the custodial parent is entitled to take the exemption. You can get no credit on arrears for the exemption.
It depends on what your court order says. If it says you shall make the claim and she won't let you, then you need to file appropriate pleadings to enforce the court order.