Alright guys. There are a couple reasons I'm posting this. First, because I'm humble enough to admit that I make mistakes. Second, because I hope that by writing it out and sharing it both you and I can learn from it.
I'm really regretting that I don't have a video setup, because this would have been spectacular to watch, but I'll try to describe it as accurately as I can.
I was driving on the highway yesterday, approaching an interchange. The entrance ramp is (I measured it with the odometer today) half a mile long, so I saw no reason to move to the left lane just to get stuck there if someone matches my speed. There was some entering traffic but I trusted them to figure it out.
As I was about 1/3 of the way past the trailer of a semi truck that was attempting to enter, it began to move to my lane. I dove into the left lane, but I made the mistake of focusing on the center barrier that I was approaching instead of the clear space ahead and swerved back to the right too quickly. The other mistake was probably honking the horn, which used some of the hand control I could have had. My car began to oversteer as I passed the cab of the truck. I honestly can't remember if I countersteered. I think I tried, but I remember reaching a certain point and thinking, "I'm ****ed, let's just ride this out."
My car rotated to the right even more and I ended up rolling down the road backwards looking at the front of the truck. I steered to the left to try to get the car to complete its rotation. By the time I stopped, I was about 20 degrees from doing a full 360 and in the middle of both the lanes.
The engine stalled because I wasn't quick enough on the clutch when I spun. It wasn't happy to restart, but it did, and I just drove off like nothing had happened... Fortunately the truck stopped, everyone behind it stopped, and even an SUV that saw it happen stopped further up the road. I wish I could attribute this to some sort of skill, but I know luck when I see it...
I think the biggest contributing factor to this situation was fatigue. I stayed up too late the night before and was still tired. Sleepiness affects both judgement, which should have told me to not trust everyone and move to the left lane, and coordination, which would have prevented me from overreacting and creating the oversteer condition.
Speed was most definitely not a factor. I don't remember my exact speed but I know I wasn't in a hurry. I was cruising at about 70 before this and probably slowed even more for the entering traffic. I also just installed new shocks within the last month and checked my tire pressure and tread depth within the last week. The car is normally fairly neutral unless provoked, and I provoked it.