I've been riding on busy roads with no shoulder since I was 7. I also use a mirror, though of course I didn't when I was a kid. Anxiety only increases your danger. Ride smoothly, predictably, and hold your line.

Two things that will help you more than a mirror are bright blinkies front and rear, 24/7. People instinctively avoid bright flashing lights. In front I have a 200 lumen white flashing light (Serfas) and in the rear a 60 lumen red flasher, also Serfas. I'm thinking I really should get a 300 lumen rear flasher, a Dinotte 300r. All my lights are USB, so no batteries to mess with as long as the ride isn't longer than their batteries will last. For really long rides I have backup conventional battery lights, almost as bright. It's critical that people see you, and having seen you, avoid you.