As an (ahem) older person who has dealt with depression and other issues my whole life, I found that I had reserves of strength I didn't know I had until the time came to find them. Many times I dealt with trials I never would have thought I could, and stay sane. I think it is inherent in our will to live, no matter how miserable we think we are, to struggle to survive and save ourselves.
No one who has not had to face these kinds of issues would understand at all. No amount of book learning or talking to MI sufferers can make a person really know what it's like. To them we can look weak and like we aren't even trying, but we have to overlook their opinions, as that is just what they are. It's an ignorant person who thinks they know all when they haven't walked a mile in your shoes.

Every day you wake up is another victory over your MI. Remember, you are a survivor.