You certainly have the ability to reach Q51 at that stage, so the difference might come down to strategy and timing. Do you notice any patterns or consistencies with the questions that you answer incorrectly? For example:
- Incorrect answers: clustered toward vs spread all over
- Specific types of problems (geometry, combinatorics, etc.)
- Rushed, particularly toward the end.
- Guess vs. "staying the course"
The first and third are related. Maybe anxiety gets a hold of you on certain questions or toward the end. In that case, you just need to take a deep breath and maintain composure. But with only a few seconds to go, I suspect something is going on there.
About guessing: do you ever guess on a difficult question or toward the end? Or do you always solve the problem, no matter how long it takes?
If you notice a specific pattern, then it's fairly straightforward what to do: focus on that problem! If it's probability, then put extra work into that. If you make a lot of silly math errors, then focus or mental energy is the problem.
And then there's "luck." Sometimes you will get some questions that just click, no matter the difficulty level, while other questions just don't feel right.