Originally Posted by dabac

With that much of an overlap, my first guess would be that you're trying to fit a 622 mm BSD (bead seat diameter - where rim and tire overlap) tire onto a 630 mm BSD rim.

If you're certain that's not the case, revise your technique. Save the bit closest to the valve to last. Start from opposite, push the bead into the center channel of the rim to free up as much bead as possible for the last heave over the rim.

And next time, try be a bit more descriptive in your headline.
Simply "help" isn't much use. "Help with tire mounting" or something along those lines makes it more likely to get good answers. And is easier for answerers too.