With the current market for engineers, if you aren't enjoying your work most of the time, you're doing something wrong and should probably find a more interesting project to work on. Fire yourself if you have to.

That said, good diet, healthy social life, and exercise are of paramount importance to will power. The better balanced your life, the more will power you're going to have.

But it's also important to manage will power. When you're tired, just stop, take a break. Even leave things for tomorrow. You are likely just fooling yourself that you can still get stuff done and wouldn't you rather just take a proper break than spend the next hour or two shuffling your feet and getting neither rested nor getting work done?

As for short breaks -> Timers. Use timers. Seriously, use a timer.

Shameless plug, I've been working on a book about this stuff: http://nightowlsbook.com