What I think people don't understand about depression is, it's not the BIG, life changing moments that always push you over the edge, it's often the little things that just keep piling up and up and you just can't get off the ride! Your mind makes one more mess from the dogs, one more late night for your spouse, one more fight with your kid just too much. So when you spill your drink, BAM...that mess is the one you cannot clean up.
It's holding onto each one of those acts and not having the ability to "fix and forget" before you take on the next moment in life. It's NEVER being able to let go & move on. A hypersensitivity to every action that is take at or by you. The bad, sad drowns out the good, the joy. And it just gets louder and louder and plays over and over! During the day at night, alone with friends IT JUST DOESN'T STOP!
And for me, dealing with this my entire adult life (probably earlier without a name), I have been lucky enough to find a way out of the dark. A husband that always supports me even when he doesn't understand. Monsters that know when I need a hug. A mother, sister and brother that have been with me through it all and never stop loving and supporting me. And a few close friends that see when I need help and stick with me for the long, dark ride.
I don't think there is ever a way to know when depression becomes dispair and I hope to never know; but I do know that understanding and support are the key to making it to the other side. Knowing that someone stands with you makes it a little easier to walk.