Unfortunately, you will not be able to get any "back pay". I agree that it isn't right, but everything is based upon prior period's income, and there's no adjustment allowed; just a correction going forward.
However, perhaps you should discuss this with your divorce attorney as if you agreed to file a joint return with your husband, he would have saved a substantial amount of income taxes based upon that filing status, while you suffered the increase Medicare premiums.
Since you are still married, you can probably use that to your advantage for the 2016 tax year by suggesting that if you agree to file a joint return for 2016, he make up the excess you had to pay in Medicare premiums because of that income that you aren't sharing in.
I would think that you would have a very strong arguing point. Also, remember that you can't be compelled to file a joint return. If you were to file a Separate return using the Married Filing Separately filing status, he would have to do the same and that would most likely result in several thousand dollars of additional income taxes for him.
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