I think I have to agree with 'acceptance'. Knowing what can and can't be changed and not dwelling on those things which you can't. I was going through a rough patch recently and was very depressed, or at least thought I was until I realised actually, no I was very sad, and the two are entirely different things. Once I'd accepted that it was legitimate and okay to be sad and somewhat compartmentalised it, I have begun to feel more in control, and being in control means that I have the capacity to find good in things that were previously veiled in a mire of feeling awful and thus have more peace of mind generally, although I'm still sad about what I'm sad about, but that's okay, I'll use it as a tool and find a way to make some good come of it.