There is no such thing as a W-2 independent contractor. That is like saying my red car is blue. Neither terminology nor employer whims determine whether you were self-employed or an employee. The facts and circumstances relating to the work you did determine this, in accordance with the law. This is not determined by the tax code, but by the common law definition of "employee" as interpreted by the IRS.

If you believe you were self-employed, you can file form SS-8 with the Internal Revenue Service and ask them to make a formal determination based on the cirucumstances. But as it stands now, you are considered an employee. Therefore, the only deduction available to you for medical insurance is the one available to all people who work as employees, which is on Schedule A (subject to the 7.5% of AGI exclusion).