How can I overcome my past failure to capitalize on the perfect opportunity?

Two years ago, after years of struggling in the post-2008 job market, I had a job opportunity that could have been the best thing that ever happened to me. It was a job that represents my values and could have brought me much-needed financial success if I had pulled it off. But it was also an extremely difficult, demanding, and stressful proposition, and I was uncertain whether I have what it takes to succeed at it. To make matters worse, when it came along, I was depressed to the point of having lost the will to live. In my bad emotional state, I was unable to go through with the job, and I let the opportunity slip. In the two years since then, I have done nothing but hold down an menial job while reflecting on the missed opportunity. I can't move on or get over the fact of what I did and have become almost obsessed with it. I need to approach the employer and ask him for another chance at it. It is doubtful that he would say yes, but I have nothing to lose by trying. However, for all the same reasons I didn't go through with it before, I still cannot work up the will to do it. Every day I wake up wanting to die and I am so depressed that I can't feel the warmth of a great opportunity; everything just seems hopeless and pointless. How can I rehabilitate myself enough approach the employer for a second chance?