What will hold my heavy rearview mirror onto my windshield until I can get it fixed?

If you look at the mirror where it was originally attached to the windshield there should be a small metal piece that you can remove. It might take a screwdriver or a allen wrench. Anyway you remove this small metal piece and then you can goto an auto parts store or Walmart or whatever and buy a kit with glue in it just for this purpose. You glue the metal piece to your windshield and when it is dry you put the mirror back on the metal piece and you have it fixed. Remember to pay attention to how the metal piece comes out of the mirror as it usually has a rounded side and a straight side orient it the same way when you glue it back on and you'll be golden.Hope this helps.

Try Gorilla glue, it holds everything and now I think they have Gorilla super glue.

You could maybe try buying a lighter one and applying it with the adhesive it comes with, or use Gorilla/Rhino glue.

You don't necessarily need your rear view mirror though.

REALLY strong duct tape might work. the gray kind. I hope it works! Sorry in advance if it doesnt, hope I helped!