1. Take a lot of practice under timed conditions. This should make you more "numb" to the pressure the clock can impose. Lots of timed practice can lead to the next:
2. Don't think about it so much. Everyone can get swallowed in despair if they sit and look at the clock and realize they're behind and think how are they're not going to finish on time and that it must be all futile! It's not. Calm yourself and be proactive.
3. You can decide not to look at the clock until you finish 2 passages, timing yourself for 16 minutes (or so) for each set. You can write this down on your scratch paper so you have a basis to remember how many minutes you should see left on the clock after you finish these sets. This way you're looking at the clock minimally, but for it to work well you must have an internal clock that tells you when you've spent too much time on a passage (this is created when you take timed practice)