This gotta be a joke..so you bought it 18 years back when you were just visiting? Been visiting 3 (only getting 30 days) times a year and no idea about visa types in Thailand?
You start by doing extensive research by typing the same question into Google. It will give you multiple links but click on the official Thailand Immigration site and it will break down all the necessary Visas available to address your age and how much money you need.
After you read it all, you can post your next question here if you donโt understand something.