Work on filling your emotional (motivational etc whatever you want to call it) gas tank and develop a process to keep the energy coming in the same as the energy going out.
How I've found that people (myself included) get into burnout is that they work their hardest on things with no positive emotional feedback. (such as completing a physics phd or dare I say programming for a startup pre-launch) Its not about toughness or anything here, that "some people just cant cut it." What is going is that the people that stay afloat have family, friends, hobbies and fun to keep them going.
What I would do is the following:
1. Identify the things emptying your gas tank (programming is extremely taxing after long periods of time)
2. Identify the things filling your gas tank (... as I said earlier ... friends, family, hobbies and fun with them)
3. Make sure you are working out, eating right and sleeping decent hours. (if your body's hormonal systems are going to be a help not a hurt they need to be taken care of)
4. Work on adjusting your balance much towards the fun stuff for a while and less from the draining stuff.
5. Slowly integrate some more draining stuff making sure the filling stuff isn't coming in faster on average.
I've burned out before, and this is how I got out of it. I wish you the best of luck here and hope to follow your way out.