"I would like to know what the heck I did in my lifetime to deserve having this tossed my way."
What does anyone do to deserve their lot? What do Anna Frank do, or the 6 million Jewish people do? What did, what did, it can go on an on, both ways.
But, I think You should consider about talking to your doctor about depression. We absolutely deserve the right to be depressed.

"I want a refund on my life."
I'm assuming we have all thought about getting a do-over. Even the person with one of the biggest do-over stories ever, Lazarus, ended the same way. He died. He still became old and lost the ability to do the things he could do in his youth. It can't be about what we cannot do; it has to be about what we can still do. I used to worry about smoking cigars. I don't anymore. I used to worry about the social ladder. Well, I still worry about it, but not as much and not in the same way.

"How do you guys come to grips with dealing with this stuff, knowing that you never really will be free of it until the day you leave this Earth?"
It isn't how we have come to terms w/ it. Our spirits our different. It is how will you. What brings you peace? What brings you joy?
Still, rail against the pain; rail against the lack of options. I wish for you a means of finding peace, joy and happiness that passes beyond the comprehension that should be experienced by those in such pain.