The best way I have found to remove the mirror glass is to use survey string. I use two strings 3 feet long. I get the string behind the glass one on each side of the round adjuster & then crisscross the strings. Have someone hold the mirror assembly while it is still on the bike. I also place a blanket on the floor. The warmer the mirror the better. Outside in the sun helps. There are other ways to remove the glass mirror but this is how I do it.

Do you just pull steadily or give it a quick jerk? I am assuming the mirror is just a snap mount into the ring shown in your picture.

 

I just pull hard until it pops out. A fast jerk might work better but the strings tend to slip out when you loosen the tension to give it a good jerk.

 

I use mule tape. Takes a fair amount of pressure.
It's also my tow strap.

 

Hey Dave, ever consider dipping the mirror in water before you pull the glass? Even better, a soapy water solution? I imagine it would lube the contact surfaces and ease the force needed.