Depression... I feel this way when I reflect on all I have failed in life... dead kittens
Oh man, I'm doing this every month or so. If I have time to sit down and think or reflect on anything I do, I just pummel myself into a hole.
Normally I just keep myself distracted so that I never have time to think, but this isn't really a solution at all, it's just prolonging the problem. Medication for depression I think, note the "IMO" part of that, does essentially the same thing. Most antidepressants, or at least all the one's whose side effects I've seen, are the equivalent of a lobotomy, chemically rendering you unable to care or worry about anything (which can be disastrous sometimes in removing a barrier to suicide). I think that medication for depression is handed out a little too readily. A good (as in one that actually trying to help you, not keep you there paying 20+$ an hour as long as they can) psychologist can do way more for some types of depressed people than a chemical lobotomy. Other kinds of depressed people I've noticed have an outright genetic rather than psychological disposition, and not just a disposition to do something that makes them depressed like I do, but an outright and direct unbalance of happy chemicals. These people I think are the ones that should be taking some kind of medication, but then if they're just on the lobotomy stuff, they have no inhibitions on following their predisposed instinct and offing themselves. Granted I've never seen an antidepressant that wasn't just the equivalent of a chemical lobotomy.
I'm rather hard on myself a lot and I've come to many realizations with this. I'll tell you right now the root of it and what they all point to if the source of your depression is your failures. You care about some things more than the thing in question deserves, your priorities are out of order or scale, you are rather self-centered in your judgment, you cannot let go, you focus on details too much, so forth. Circle any that apply.
What really helped me is observing other people more often. I critique myself a lot. Perhaps I make a stupid mistake, I'll spend the next half hour being the silent person in the corner, while grinding myself in my mind. What I've found is that in that half hour, provided I'm not too busy grinding myself for being stupid to notice, there are at least half a dozen mistakes made by other people around me. However, usually I'm too self centered in my judgment and critique to even notice, and so it becomes a bigger deal for me than it really should.
As for you, there's nothing specific I can say. If I was saying this to someone I've actually been around, like a friend, I could be helpful (I'd hope anyway), but this is more or less a hopeful rant.
not having anything I can do about it really tore me up
Helplessness pisses me off, and depresses me which usually leads to a rather odd set of feelings. The only thing I have to say about it is that you have to learn to accept helplessness when it happens. Kind of a life lesson, especially when concerning death. Quite simply, if there's nothing you can do, then there's nothing you can do. You're not at fault in such cases. You'll always feel guilty somehow, but when shit happens regardless of you, then shit happens.
past failures (and grades)
This is the one I have trouble with. Unlike Runt, you know you had control over what was happening at the time. As for me, I remind myself that when something is done and unfixable, then it's done. I have some trouble accepting the helplessness of the fact that there's jack I can do about something, but I ended up reminding myself of a small argument so I don't brood over stuff like I used to: Once a mistake is set in stone, it's another mistake to waste time over it. You just have to let go and move on.
Hope something I said here can be useful somehow.