Your vocal mic is too far from your mouth.

Yes, that’s the main problem when you’re trying to sing soft and the guitar is loud. The quieter you sing, the further back you tend to lean—and then the sound guy turns you up, and the mic starts eating your guitar.

Feedback happens because the mic hears the guitar’s soundhole better than it hears your lips. Fix: get right on the mic, like an inch away, and point the soundhole of your guitar away from the mic capsule. If you’re using a big condenser, switch to a cardioid dynamic like an SM58—they reject side noise way better.

Also cut some low-mids on your vocal channel (around 200-400 Hz). That’s where acoustic guitars rumble and excite feedback. And if you can, move the monitor so it’s not facing the guitar’s body.

You can have a loud, percussive acoustic and still whisper-sing, but you have to work the mic like it’s a tool, not a prop.