I don't find mirrors helpful at all...there is almost always traffic behind me, wasting time and attention looking at it isn't helpful to me. I can turn nearly 360 when I need to see what's behind me. Chose good routes...avoid sharp turns, especially on steep hills. Avoid tricky merges...like ones near highway exits/entrances. Don't be afraid to get off the bike and walk if the section scares you.

Always be on high alert around cabs...they act unpredictably. Stay away from rental trucks as the drivers are usually moving...which is innately stressful and distracting...and lack feel for the large vehicle. Make sure you're bike is in TOP FLIGHT condition, especially the brakes. Don't ride faster than you can safely stop. Always be aware and assume people might behave unpredictably. If the shoulder is too narrow, the curve too sharp, or you're near problem areas, take the lane. Get out when it's safe, but don't be afraid of taking a lane the times you have to. Or walk if there's sidewalk.

I don't ever remember being afraid because I've been riding my whole life, or as far as I remember...but it's probably just time in saddle. Above all, ride predictably. Let cars know what you're doing. I honestly don't have that many problems, and when I do, I am usually in control enough to mitigate risk.