Prevention of global warming or Venus terraforming

This maybe a rather stupid idea but would it be possible to place a
very large sunshade between Venus and the Sun.
I am imagining an ultra thin reflective surface placed in a stable
orbit position around the Sun. Maybe a mylar disc slowly spinning to
keep it stretched out.
Would the light push this like a solar sail or could it be positioned
so that gravity cancelled this out ?
To kill two birds with one stone this could act as a mirror aiming the
suns rays at Mars.
If this idea is feasible the same technique could be used to prevent
Earth suffering from global warming. The disc would not need to cover
the whole Earth, but just enough to cool key areas possibly reflecting
the light to provide longer hours of daylight in the North of Canada
or Russia in winter.