How does an instructor not paying attention cause you to almost wreck your car?

He assured me I had enough time and space to pass a slower car in front of me at the end of the straightaway going into T1 at Summit Point Main and told me to go to the outside. I ended up side-by-side through the turn and too far outside and had a nice off track excursion. I like making decisions for myself, so I've stuck to karting and autocross.