Hi and thanks for your eloquent comments.  I can't agree more that what is frustrating about your depression is that there is no clear explanation for it.  I find that mood disorders are like any medical condition, which we like to explain if possible.  If someone has a heart attack, we often observe that they smoked too much, or didn't exercise, or had a strong family history.  The way most psychiatrists understand it, usually there is a trigger or stressor which sets off an episode, on top of an underlying vulnerability which is more likely biological or genetic in nature.  I am also speaking as a practicing psychiatrist with my own mood and anxiety issues, though my low points usually do not last that long.  Then there is the impact of the depression, which usually leads me to stop doing much of anything other than work and essential tasks.  This can lead to isolation and withdrawal.  Then there's the guilt that I'm not really appreciating my work or family, as you mentioned.  At times medication has been helpful but I haven't stayed with it for some reason, maybe ADD or pride? 

in your case I would suggest either getting free brand name wellbutrin if that helped, or considering a treatment for TRD or treatment resistant depression.  this might include using abilify or seroquel with the wellbutrin, or possibly another medication called symbyax.  these medications are proven to help with many such conditions, and also seem to have a mood stabilizing effect.  I hope you can find a prescriber who can work with you to find such a regimen.  of course, the side effects also need to be managed and that's where I tend to earn my keep in the office.  Not to be too one dimensional, since a good therapist will enhance the medication piece immensely.  I would consider cognitive behavioral therapy, since that can yield more immediate results. 

good luck with your treatment and keep us all posted!