Suffocation No Breathing, or: How to Succeed in Business Without Really Trying

Literary agent Rachelle Gardner says that though platform is important, writers should only spend 10% of their "free" time on social networking/platform building, and 90% on improving their writing.  But which 10%, and which 90?

Since the school year ended, I've been spending a huge swath in the middle of the day doing my part-time job.  That swath is necessary; I've got to pay my bills, and my savings is pretty pathetic since I 1. visited family 2. bought a new computer (my old one died).  But that swath is also supremely tiring.  Though I only earn a few hundred dollars a week, my job is draining, and I've been having trouble writing as much as I should.  

Not timewise; I spend quite a bit of time staring at the computer or making notes.  But I'm so exhausted that work is slow and painful.  It feels like a chore.

What to do when the job gets in the way of the career?  I feel as though I'm going crazy; this is why I quit my last job.  I *can't* not write-- I get cranky and angry and frustrated and sad.  

But I'm just plain too tired.  Tips please?