One thing all Time Lords know is that messing with timelines is dangerous. It’s one thing fixing events so they happen they way they’re supposed to, but a very different matter trying to alter them to get a different outcome. Changing the past can have disastrous consequences for the future.
But the Doctor is perhaps a little unhinged, he’s been travelling alone for too long, and he doesn’t want this situation to play out the way it’s supposed to. He can save Adelaide. He will save her! He’s unstoppable, the Time Lord Victorious!
The past resists change.
Adelaide dies anyway.