How Could I Be So Stupid/Careless?

I was 19 at the time, my girlfriend wanted to go to Toronto to visit a friend, plus I had a friend who had recently moved to Southern Ontario as well. We lived in Winnipeg at the time, and the trip was about 1300 miles each way. We got to the Toronto area no problems, but as we were leaving, the differential in my 74 Ford Torino started making some bad clunking noises.
Being 19, I didn't have much for resources, no credit cards or such, nor did my girlfriend, she was 17, 2 months shy of 18. We decided to take the chance and hit the road, driving cautiously and not pushing our luck.
We made it about 800 miles or so before it got really bad, the gears would freewheel several times before finally engaging and the car would move again, and I was worried about it quitting completely in the middle of nowhere, which is pretty much what Northwest Ontario is for miles.
My girlfriend's friend had family in a little town called Nipigon Ontario, and they were kind enough to let us leave the car in their backyard, and we hopped a Greyhound the rest of the way home to Winnipeg.
Over the next week or so I bought another car, loaded up my tools and another complete 9 inch differential, and my buddy and I set out to retrieve my car. We had both worked all day, plus he did a cam swap on a sbc after work, which I helped him finish, then delivered that car to the owner, and finally got on the road about midnight.
We drove the 500 plus miles to Nipigon, put the Torino on stands, pulled out the grenaded center section, and all the shrapnel that had been spider gears, reassembled everything, and were back on the road by early evening.
About 50 miles into the drive home, I'm doing about 70 mph in the Torino with my buddy following, come around a right curve, and boom! The *** end of the Torino jumps about 4 feet in the air, slams down onto the pavement, and I have no left rear tire. Car is skidding around, sending a roostertail of sparks in the air, and I managed to get it stopped on the shoulder.
We realized that we each thought that the other one had tightened the lug nuts, 2 were broken, 1 stripped, and 2 were usable. We popped 3 studs out of the spare axle in the trunk of the second car and raided one lug nut off each wheel.
Never did find the missing tire, guessing it went a fair distance into the bush, put the spare on and continued home, non-stop.
By the time we got home, we had been up for some 56 hours straight.
I'm still very thankful that there wasn't a vehicle coming the opposite way when that tire let go.