Help needed with door mirror glasses

[T_AE86]

Hi,

I decided to paint my door mirrors and grinding the edge of the mirrors, but when i tried to mount...i have a problem:

I have to get the "ball"...




...in the hole



I lubricated with silicone oil and oil, I tried forcing the clamping (like picture below) but is imposible think I'll break the glass over tightening. Does anyone know how to do?...without breaking the glass



i don't know if take forcing a screwdriver forcing to open the "hole" for a night will help (as in the picture above)

Thanks