1. Stop taking about it on the internet, this is not confidential.
2. Talk to a tax attorney or CPA about the situation, you might have to pay back the IRS. They will be able to better advise you.
3. Don't pay the extortion money, because she will just keep coming back to that well.
Your tax issue is more important than the attempted blackmail. If you resolve the tax issue the blackmail is moot. As stated above speak to an attorney and get everything squared away with the IRS. Also, be conscious of the impact of your admissions...internet material is visible to anyone and everyone.