For tax purposes you're either a dependent for the entire year or none of the year. There's no such thing as a dependent for part of the year.
If you became ineligible to be claimed as a dependent in 2018, you are responsible for your insurance for the entire year. If you were covered by an ACA exchange plan for part of the year, even if somebody else was paying for it, you would still enter the insurance information related to your own coverage on your tax return.
