I think a made a big goof!

I was trying to wire a light switch to an existing ceiling light
fixture (pull string) in an unfinished room. All I did was run the
standard 12-2 wiring from the new switch box up to the existing
fixture. Then I drywalled everything up. I went to wire the switch box
line up with the existing wiring that leads to the fixture. When I
turned things on it tripped and won't properly work from the switch.
The pull string works fine (off and on), but when I try using the
switch it trips that circuit.

WHERE I THINK I WENT WRONG:
I now think that I was supposed re-direct the existing power that ran
to the existing fixture in the ceiling down to the new switch
location. Then run a separate line of 12-2 from the new switch
location up to the light existing light fixture. Am I right? If so, is
there a way to rectify this without ripping out the drywall to have
the switch and fixture work properly?