you're killing me

12mph is too fast for you. I can't even imagine how out of shape you must be if you can't ride 3 miles on the first attempt. My guess is you went to max heart rate rightaway, and your body panicked. 12mph in low wind conditions is about 130W of power output on a mountainbike. Walking briskly is about 100W, so you should aim for that until you feel good about it. That means no more than 10mph, less if there is wind (even a sidewind slows you down).

Ride safely without distractions (dedicated bike path if you can) until you are absolutely at home in the saddle. You can die easily in traffic if you can't panick stop without going over the bars.

Good luck!