Oof, I'm sorry to hear that. If your ex applied for a credit card in your name without your permission, that is technically identity theft. There are a few steps you can take to get your credit back ...
Credit cards have become an insidious yoke of debt around the necks of millions of people, especially usury levels of 20% ...
When I turned 18 (25 years ago), my parents had me open a credit card in my name so I could start building credit. I never used it though—they did (responsibly, this isn’t going in that direction).