I hope this helps
Any person born in Malta between 21 Sep 1964 – 31 Jul 1989 automatically get Maltese citizenship at birth. BUT you would have lost it when you turned 19, because you didn’t renounce your British citizenship as Malta didn’t allow adults to possess dual citizenship.
However, the law was changed in 2000 so former Maltese citizens by birth or descents who had lost our can be be conferred with citizenship retrospective to the date it was lost. Bascially, you never to lost your Maltese citizenship so in theory you are already Maltese.
You will need your Maltese birth certificate to apply for your passport. Visit the Maltese Government website for more info and to apply for your passport. https://foreign.gov.mt/en/Pages/Home.aspx
It should be straighforward