I know it's hard to stop answering a question when you want to go into detail or know more about it, but before the exam work out the timings for each exam. This is easier in exams with a few questions but maybe for things like science my exam was in modules so I figured out how much time needs to be spent on each section. If for a 5 marker you've worked out beforehand you can spend 5 minutes then ALWAYS STOP AT 5 MINUTES, if by not completing that extra 2 sentences you lose a mark or 2 then so be it. This means you'll get to attempt all the questions and it's likely that you'll have a couple of minutes to spare here and there because maybe on another question you didn't quite know the answer so in this time go back to questions and add on anything else. You're better off losing a few marks by not completely finishing questions than losing a whole load of marks by missing off the questions at the end which are usually worth more anyway.
If it's nerves like you said then go into the exam, read through all the questions and tackle them from best to worst or the other way round depending on which way you feel better with. Going from best to worst means that you build up on your confidence on the first few questions you answer and you'll be in the exam mood to attempt the more difficult questions. Also, it might be that the first few questions could trigger something and you might remember something intitially you struggled to come up with.
I know it's a bit long winded but hopefully it helps! I think my school has just drummed into me how to sit an exam it's just normal to me calculating times beforehand and actually sticking to them in the exam. Good Luck!