It's interesting that your presentation, and in particular the process you went through to choose the topic of it, is what you remember most vividly.
My only explanation is that something happened as a direct result of that process which seriously compromised the integrity of the timeline you occupied before the switch, so your consciousness made an effort to go back as little as possible and change as little as possible while still avoiding this sensitive topic which, when touched on, ruined everything.

The reason I think this may be the case is because I experienced a very limited version of this as a kid. I relived a normal school day, because at the end of the day, my family and I went to watch my older brother play in a baseball game. While he was playing, I was dicking around with my twin brother like I always do, and we climbed to the top of one of the tall-ish fences the surrounds many sports fields. I was sitting there, just enjoying the view and the late afternoon sun, when I lost my balance and fell forward.
Everything went black just as I hit the ground, and all of a sudden I was waking up in my bed at home, like it was just a bad dream.

At that point, I relived the 'dream' I just had.
Everything besides the ball game and the fall was so normal and identical to the 'dream' that I don't even remember anything about what I did that day besides the fact that our family went to that ball game and I climbed that fence.
But the second time, I didn't fall off. In fact, it was at that second moment on the fence that I remembered my 'dream', and realized that I'd actually relived the entire day, right up until that point.

After spending some time on this subreddit, I've heard people tell of their versions of this same experience.
It's always pretty much the same: A person (or sometimes a few people) will get into a situation where they get into a bad car accident or get assaulted or drown or something else bad... and then one of three things happen:
1. They go back and relive some amount of time leading up to the event, and then somehow avoid it.
2. They quickly, in the exact moment of the event, jolt 'sideways' into a version of the situation where they miraculously didn't die.
3. They fall forward in time, often times finding themselves in exactly the same place as where the event occurred, except that it is some time after they remember the event occurring, and they have no recollection of how the time in between passed.
The second and third options usually manifest during some kind of violent car accident, though this is certainly not always the case.

Surely these people could be lying, but based on the consistency of the elements in each story, combined with the personal experience I had, I'd say that this is proof that your conscious experience can and does shift away from realities that cannot support it & into a reality that matches the one before as closely as possible.

This does raise the question of how a class presentation could kill you, but I have a theory. It seems like in the previous timeline, you were so intent to doing a serious paper about drug abuse that you were committed to gather personal anecdotes to compare to your own live.
This is a long shot... but it could be that in the process of doing that, you ended up spending too much time with the wrong people, and you got tangled up with them.
Perhaps you started doing drugs again. Maybe the empathy you had for their suffering pushed you so far into depression that you ended up killing yourself. It could even be that you investigated too much and they took you out when you learned something they didn't want you to...
Obviously, I don't really know.
But to me, that would explain why in your current timeline, you "played it safe" and did yet another presentation on the familiar, innocuous subject of family history.
In this case, it seems as though it literally saved your life.