The reason of your problem is that there is a spring inside the axle of the mirror which presses the head of the mirror with the base.
When the spring loses part of its strength, the mirror continues opening until it is stopped by an obstacle (the door in this case).
If you stare at the axle of the mirror (without the cap) you can see a small step where the mirror should be blocked when it is opening.
I know that a friend who had the same problem tried to fix it inserting a steel washer (which was cutted until it looked like a 'C' letter) between the spring and the top trying to tighten the spring a little bit.
Maybe you could try to clean and apply grease the spring in order to get it sliding smoothly.
The other way is replacing the mirror for a new one... but it is expensive...
PS: Let us know if finally you are able to fix it.