Found this:
Rear view mirrors are commonly secured to windshields with strong adhesive so that they don't fall off in an accident. But when it comes time to replace that mirror, it can seem like it's a bit too secure and difficult to remove, at least...
www.wikihow.com
I would suggest a hair dryer or heat gun...
The glue on these is pretty tough, it is doing a great job sticking to s smooth surface.
Heat will be a challenge for most glues and it usually causes it to weaken if not release completely.
If you are not happy trying this, talk to a windshield shop - they can advise and have likely done this.
Good luck and keep us posted!
Clive