Narrowing the scope of what you need to accomplish is extremely effective. The best advice I've ever gotten for dealing with depression is to just do one thing at a time.

For example, if I'm sitting in a chair and I need to do something, I only need to try standing up. I can always sit back down if I don't have the energy to start walking. Most of the time, I keep going. The times I don't, well, I know I tried.

If you can define what trying looks like, then you know when you've tried your hardest and failed through no fault of your own. (For most people, it's a lot more than just five minutes!) Accepting failure makes moving forward in another direction a lot easier. Guilt isn't a good emotion to dwell on.

The important thing with this advice is to remember you probably don't know your limit. Most people keep more energy in reserve than they realize and they can afford to spend that occasionally. This means the advice to push beyond your limit is really good advice for most people. But for many people dealing with chronic health issues, we already tend to push beyond our limits and hurt ourselves. Thus we need the opposite advice. :)

Edit: I should clarify, by "dealing with depression", I mean trying to be productive when you are depressed.