Mount the mirror on your helmet, not on your bike.
A helmet mirror will have a wide field of view, be large and easy to see, and can be aimed by moving your head to see anywhere needed. It is right there adjacent to your vision and your brain will learn to see things in it without you consciously having to look. It also does not suffer from vibration.
A handlebar mirror requires you to look down and peer at a small image on a vibrating mirror that is itself pretty far from your eyes. It sucks and what sucks even worse is crashing into a parked car or pothole because you're trying to look in the mirror instead of at the road.
Put another way: a helmet mounted mirror is like a HUD, a bar mounted mirror is like a display somewhere low on the instrument panel around your knees.
I actually use a mirror that clips to my glasses, which is even better than a helmet mirror (even closer to your eye) and this model happens to be real glass, optically excellent, with a sturdy brass arm. Bike Peddler Take A Look Cycling Eyeglass Mirror (Compact)
https://www.amazon.com/dp/B000AO7ETQ/ref=cm_sw_r_cp_api_i_MQQcEbKBKJMKV