A supplied-air respirator is the only way to safely spray modern paints using hardeners. Some individuals may have a higher tolerance for the adverse effects of isocyanates in hardeners than others. Not everyone is the same though and like you, I learned the hard way many years ago that a cheap mask, even a charcoal one, was NOT sufficient. Painting outside is not a solution either. I have always done all my painting outside. Not long ago I repainted the moldboard of my Fisher snowplow - also outside. Because of the curvature of the moldboard, I was totally enveloped in overspray being redirected directly into my face. I could barely see where the paint was actually being applied, only 2 or 3 feet away. My full-face supplied-air respirator protected not only my lungs but my eyes also.
Unless someone is required by employment rules, using a supplied-air respirator when spraying paint with hardeners on a DIY basis is a personal choice, as it should be. No, the equipment is not cheap, but it's always best to minimize a real risk to your own safety and good health. Using hardeners is, for the most part, optional for enamels except for the urethanes. My advice to anyone contemplating using hardeners in paint is to get the proper supplied-air equipment or simply don't use optional hardeners (or urethanes, which require such additives).