Some musings of my own follow, mostly about my own insecurities. Don’t assume that they are meant to apply to any particular person on this forum (no, not you either, person reading this right now).
Mental illness comes in many different forms and they are all too often conflated, because none of the words we use to talk about mental illness have any kind of clear meaning.
There are different kinds of depression. Some are simply caused by environmental problems, such as nutrition, or chemical issues in the brain. But not all of them.
The one that I, personally, have experienced (probably for around 5 years now) can be described thus:
It is the conflict, deep and painful, that you feel when attempting to convince yourself of something that, on some level, you don’t really believe.
It’s a closely related concept to akrasia, as well as to procrastination, but depression is different. Depression comes with a degree of suffering that does not come with those other things.
Willpower is the ability to actually convince yourself of something, so that you really do believe it. Or at least to force yourself to do it even though you don’t.
Willpower, however, is overrated. The problem with willpower is that if you’re not careful you’re just as vulnerable to convincing yourself of something that is false or dangerous as something that is true and good. Willpower can lead you to achieve great things, but it can also lead you to being a Nazi.
Having a “lack of willpower” is what society accuses you of when you refuse to conform to society’s expectations of you. Quite a lot of the time—most of the time, even—society is correct. But sometimes society is deeply wrong, and in those instances that so-called “lack of willpower” is not a negative trait, but a uniquely positive one.
For those instances, the key skill you need is rationality—the ability to actually figure out whether the things you want to convince yourself of are really, genuinely true.