I definitely understand you. The reality is, although I don't advocate it, deadlines at A Level sometimes have to give. My biggest problem has been that teachers, even in the same departments do not co-ordinate with each-other. So if you have three teachers for one subject, as I do for two of mine, all three of them very often give essays at the same time etc;. What you need to do, is set yourself 'mini goals'. Looking to the end of year and university is good, it might help you, but it will help you if you focus on smaller motivations and goals. For example:
Is your next essay in the immediate weeks before parents evening? There's a reason to stay motivated.
Is your essay the last one of the term, and therefore the last time before your next report to prove your potential to your teacher? There's a reason to stay motivated.
Do you have a teacher who thinks you can't do well. Stay motivated and prove them wrong.
Are you happy with a C in your coursework draft and are willing to just hand that draft in as your final one? Think that it could be moderated down to a D, and you need a B to be sure of your final grade. That'll keep you motivated.
Basically, my point is, find small goals, small motivators, don't just think of working your ass off to get good grades in the end. It might work but you'll go crazy if your motivation/goal is to do well in something that is so far away (chronologically). And really don't worry too much about missing the odd deadline by a few days now and then. Sometimes sanity has to take priority.