Mental illness, especially depression, is not something you “get through” or “get over”. It ebbs and flows, waxes and wanes, but it never goes away; it’s always there. The best you can do is learn to cope and function in spite of it, to work around it. Some of us never make it that far, and some of us do but then the illness changes, and we have to start all over again. It's a constant struggle, day by day, though it is easier on some days and harder on others. Even the best of us have times of darkness, and sometimes that darkness is just too much to bear.
Do not look down on those who fall along the way; they were not weak. Every day that they survived showed their immeasurable strength. How their journey ended does not lessen that.