<human>: This fuck up occured 15 years ago in High School when I was in black and white photography class. We had a  blackroom to develope our own films. To develope a roll of film, we'd take an empty canister from the countertop sinks with us into a pitch black room and insert the film into the canister to prepare it for developments. 

My fuck up occured because I didn't pay enough attentions in class on the locations of empty canisters and putting canisters with the film inside ready for developement into a class assigned cabinet. I had to put the film into the canister, so stupid self went to get the "empty" canister from the cabinet, opened it up to discover a roll of film inside. My reaction? Who was this stupid person to misplace their canister? I told my teacher about this, she was baffled, then expressed horrors when she learned I got it from the cabinet. My heart stopped, I wanted to die really bad at that moment.

Then this girl was looking frantically for her canister. She was pissed and equally upset. I manned up to her that I was responsible for her horrific circumstances. Her face was nothing but anger and loss. I think I might have made her cried too. I was so distraught, the heart sank further than the Titanic could, and I wanted to die even more, I just wanted the world to end at that second, sacrificing humanity to save myself from sheer embarrassments. My whole body literally shook violently, I hated myself, I could not function that day, I was having an anxiety attack in the pitch dark room as I was loading the canister with the film. The darkness did not help me at all.

Can you condense the text into a few key points?
<bot>: I got confused with class assigned cabinet film loaded canisters for empty canister, opened it up to ruin classmate's undeveloped roll of film.