Sometimes you can force yourself to complete the task, with a crack of the determination whip. But there are other times when, no matter how hard you try; you are unable to muster the creative juices necessary for the literary enterprise. Sure, you could plod along, but the thrill is gone and you will eventually grind to a halt.
When I first started writing, my ideas were big and epic! I was young and craved the excitement that only comes from the most outrageously unmanageable notions. I finally exhausted myself and ended up focusing on writing shorter works. I did eventually finish the novel, but it was years later...
If you have taken in hand to write a massive work, remember to keep your goal in sight. It is the destination you are heading towards, and be aware there will be quite a journey to reach it. Mentally prepare yourself for the task ahead in the same way you would pack plenty of provisions, proper clothing, and suitable gear for a long trip.
There are many tools to assist you: an outline, character profiles and even a synopsis. If the story is large, then the more you write about it the clearer it will be when you actually write it. The characters will be real, because you already know them.
One thing I have learned is that every story need not be an epic. There is a satisfying quality to a short story: they are brief, but enjoyable; they do not drain you quite as much; the idea is simple and the gratification immediate. There is a time for all things, but before you take in hand to construct a wonder, make sure the building materials and workers are close at hand.